yep that's sounds close to how the maths work. you are obviously already better than me at the maths, 
Just remember that this only tells you what the rolloff point is at that point in the circuit.
By the time it gets to the speaker it will have altered, often by a large amount.
Much like the Duncan tone stack app as a good example, Yes a big help but only tells one part of the whole system tone shape.
Phil.

From my experience a lot of Hi-Freq hash comes After the pre stages. I've found a lot of hi freq crap can be removed before power stage which in most SS amps runs clean.

I would try adding a higher value cap at C26 at U5B (Preamp output). It's 22pF on the Deluxe 112 Plus Schematic I'm working from which will likely be similar as most of this era SS fenders use very similar circuitry so just find the Preamp output section. 
You can just leave the 22pf in place and bridge with higher values until you find a sweet point. You may even need 1nF.

The input section on most of this era of SS fenders are close to identical, it's just a hi pass filter which is helpful but it and the rest of the circuit does not have enough suppression of the high frequency,, so the result is a very harsh treble so when in distortion modes they sound crappy and harsh.
I resolved this crap hifreq issue in a Fender Performer 1000 by doing the above and swapping to a darker speaker.
I doubt these amps will ever win awards but you can improve them somewhat.
Phil.

It's a long shot but,,, maybe check you have continuity from Mains plug earth pin back to chassis. For that matter try another wall outlet they can indeed fail on rare occasions? (if earth pin in an outlet fails equipment still works,, just no earth )
Phil.

Here is my edit of Epi-Regent schematic, there may well be things I've missed.
Adding a screen shot of the tone curves.
Note; looks like C11 is 220nF which seems to work in the Simulation.
Screen shows 3 plots;
Yellow trace; Treb 10, mid 1,  Bass 10
Green trace;  Treb 1,  mid 10, Bass 1
Blue trace;   Treb 5,  mid 5,  Bass 5

Could be improved if Mid dip was shifted closer to 400hZ
Phil.

When you reinstalled the PCB you may have inadvertently disconnected the Chassis Com from circuit com.
If the amp is otherwise working I would be checking that you have Circuit common in contact with Chassis.
If that connection is open it will cause hum.
Phil.

I said,
Re R5,R6 and C6;
These are not in the signal path, R sets the gain of the stage and C sets the hiFreq roll off.

*This is the FEEDBACK path Not Signal path.*

You said:
" In this case it looks like it's bleeding it around IC1-B (?). "
YES Correct

C6 basically dead shorts hi freq (set by the value of Cap)
So the stage does not amplify above a set freq.
Higher values will wipe off at a lower freq.

Valve Amps used to have a Presence control built into the power amp stage.
Simply by adjusting a pot which progressively added or subtracted a Cap in the FEEDBACK path, More cap less treble.
Nothing magic Just another place to alter tone.
------------------
If the tone knobs can't give you good results then it's a matter of tweaking what I call *Whole System Tone*
The feedback path is often used to do this.

Side note;
 I did try to simulate your tone control setup but it does not seem to work well.
I can't read some of the values so I guessing.
If you can give me the right values I maybe able to work it out. 
--------------
Something that is often missed ,,well at least I did when learning how this stuff works, LOL;
When you look at a schematic you are actually looking at 2 circuits intertwined on one drawing.

There is the AC path, (signal you hear) Which floats on the DC potential which comes from your DC supply. You have to set the DC points so as to pass the best signal. 

As I mentioned before just Google stuff.
Also down load a sim program and just fiddle with circuits relating to your Q's.
Often sims come with a library of circuits to help you get started.
Meantime trawl through old book stores you may find some electronics books.

I believe you can download Art of Electronics in PDF.
I have that book and it is not too hard to grasp.
How fast you learn depends a lot on what you already know.

For me it was a lot of reading, and Bread board testing.
But once I used sims my understand went ahead very fast.
When I build/design a circuit I simulate it while also breadboarding it. The advantage is you not only see a plot on the screen but then you can hear it live.
Sims are not perfect but seeing and hearing at the same time speeds up the build process big time.

with BBoard and Sims you are able to tweak stuff to taste as well as find all the problems before you commit to a PCB. 
Phil.

@ FenderD112Plu, Regards Test point 7 & 8
Those AC test points need to be taken with a signal at input.
Read Note 5 bottom left corner for clues.
Phil.

Welcome Marcello,
My guess try this;
Remove C21 (lift one end to test)
That will remove some hi Freq

Also you can raise the value of C25 (Add 5 or 10uF there)
That Should (hopefully) add a lot more bass.
those 2 mods will help balance the system tone shape.
Phil.

Quote from: SemiConductive on April 10, 2022, 02:47:27 PMUpdate on the hands-on side: Successful test!

I lifted the diodes from ground and put a toggle in there so I could do back-to-back testing.

The diodes were definitely clipping at fairly low signal levels... as soon as the gain pot was set to above minimal. By disconnecting them with the switch, I can get a lot more clean headroom, which is what my goal is for this amp... and as expected.

Turning on the "overdrive" switch in the panel and bringing IC1-B into play bumps the volume fairly cleanly. But with that on and the gain turned up high you still get into distortion territory, again as expected.

There is a little bit of a treble cut when you switch on IC1-B, even at low levels where it stays very clean. Very slight cut, just taking off the upper edge a little. Not unpleasant but a minor change. Perhaps the resistors and caps hanging above IC1-B are doing some filtering when it's on? Or maybe it's the opposite... there's filtering when the signal runs fully through the resistors/caps above with IC1-B shut off and turning it on eliminates that? I'm a little confused by the fact that the caps appear to be "in-line" (not going to ground) and I would think they'd filter bass, if anything, in that configuration.

Good to hear,
Yes at higher levels the power chip will distort,, as well the power rails might be sagging a little bit due to the bigger current draw.

You can add a small value pot in series with the diodes if you want more control.

Re R5,R6 and C6;
These are not in the signal path, R sets the gain of the stage and C sets the hiFreq roll off.
you could lower the value of C6 which may balance it out a bit better but if you want the same freq response then you would have to switch the cap value to match the resistor change.
Likely not worth messing around with it.


Just search with *Opamp basics* or something similar as there is a stack of info on the web.

Here is a site with a massive amount of info on audio electronics.
https://www.sound-au.com/index.html
Click on *Articles* and search those for info.
Some of the *Projects* also have a good explanation of how they work.
Rod has written this site for folks like you (and Me ) with limited knowledge.
Phil.

Quote from: Enzo on April 10, 2022, 03:54:24 PMI stand corrected.

Well that's a rare event for you
Phil.

Quote from: SemiConductive on April 07, 2022, 12:06:30 PMYeah, this is actually the schematic from the manufacturer too. LOL.

From what I see, you are correct. Pins 3 & 5 are positive inputs. Pins 2 & 6 head to ground.

You're saying they'd normally flip the positive the other way when using both halves of the 4558? I'll look when I'm in there in a few days to see how it's really wired. 

Pins 2&6 are Neg inputs,, just don't assume that because they are Negative they go to ground.
As a rule of thumb the Positive input always needs a ground reference, often a resistor.
If you notice the Pos on IC1A has a resistor to Ground/Common.
While IC1B the Pos input goes directly to Ground/Common.
These chips are nearly all differential inputs, IC1A is non inverting while IC1B is wired as an inverting stage. both need the Pos input to have a DC path to Common.
Phil.

The circuit is drawn right but labeled wrong, just to clarify a pic may help;
You cannot view this attachment.

You may only need a low value pot for the leds ~ even 1k would likely be enough but use the lowest value you have on hand.
Yes odd ball tone design but if it works then leave it.
Phil.

I doubt you will blow the power amp chip by removing the leds.

I tend to think these little chips blow because quite often the modern metal shred players use very high out put pu's and thrash these chips to death. Often adding pedals to make it even louder.
Add to this it's a small amp and more often than not they have tiny heat sinks which are not up to task, so the combination of above will fry the chip.

At sane bedroom levels which these small amp are designed for you won't have issues.
So As Enzo mentioned lift the 2 Leds (or one at a time) and you will get more clean head room but as it's a small power chip it may not be much more because the power section will distort.

you can if you wish add a pot in series with the leds and that will allow you to dial in how early the leds clip.
(Leave C9 bridged from R13 to ground)

You could try better speakers but that will get expensive and still may not improve the SPL by much.

If you want more clean head room then you will need a bigger amp. Sorry
-------
Regards the C9 Q;
The cap is there to wipe off excess hifreq crud.
Valve amps wipe of hi freq by design while SS amps you have to add some hi freq roll off.
A lot of budget small amps don't bother to tweak this issue and hence sound very harsh Especially when in distortion mode.
Again lift it and hear the difference.
You will certainly have heaps more treble.
If the Amp is already bright then Leave it in place and just lift the Leds.
If you want, you can try different values on C9 to fine tune the top end. (lower values = more treble. higher = less bright)

One thing that may happen when you remove leds and engage the distortion.
The signal going to the reverb may start to distort the reverb drive. so you might have to raise the value of R201.

Just a note on the schematic;
Unless I'm missing something??? IC1B is labeled wrong.
Pin 5 is the Positive input and is normally grounded in this configuration,, It looks like it's + & - input labels are flipped. I can't read the label Numbers,, they both read 5 to me.
Hope it helps
Phil.

No worries chum,
 Just a couple of things that may help if you try this.

1/ The Master out has a very simple level shift foot switch. In use the trick is to adjust the master to balance the amount of volume lift you need.
Should be obvious that with the master full up there is little difference in volume, while with master at low level the volume jump is likely too much.
You just adjust the master to a point where the level shift suits your need.

2/
If I was to build this again I would change the switching sequence so that the trim section remained on all the time.
You just need to reconfigure the position of SW2.
3/
Depending on how bright or dark your Guitar/Amp system sounds you can play around with the cab sim values to fine tune it.

R3,R43 are there to drain the caps when switching.
Also add them on at C20 (switch side).
Sorry forgot to add them to Schematic. I think I added them on S3 as well. I forget  exactly where I put them but I assume most folks will bread board this before jumping in.
One day I'll try to get a video of my gear in action but my little camera has built in compression on the audio which does tend to slew what you hear.
Phil.