I bought a used Marshall MG10CD... yeah, I know.
I guess it's one of those pieces of gear, like a DS1 or Metal Zone, that you need to have owned to understand all the hate.  <3)
I have found the "10" stands for: needs 10x less gain, 10x less treble, 10x less output and 10x less hiss.  :duh
I'm planning to mod it and turn it into a head with a Celestion fitted cab but I need to tackle some issues first.

Problem 1: hisssssssssssss
I can probably fix the gain and treble/voicing issues but I'm really struggling to figure out where the hiss (white noise, not hum) comes from. I see other MG owners complain about the exact same issue.
I understand that with lots of gain, there will be hiss but the problem is that even with nothing plugged in, there's quite a lot of hiss. With no gain other than coming from the power amp I don't understand where this hiss comes from.
At the levels I'm playing, turning up the gain to 10 only adds hum, and pretty much no extra hiss.

There's a pretty good analysis and nice schematic of this amp here: https://www.electrosmash.com/marshall-mg10
But unfortunately he made some mistakes when tracing, especially in the tone stack and Sallen Key sections, so the frequency plots are wrong.
R18 and R19 are 10k instead of the 100k in the schematic and he's missing R12 47k to ground right before R13

Link to the schematic in case the horizontal scroll bothers you:
https://www.electrosmash.com/images/tech/mg10/marshall-mg10-schematic-parts-big.jpg



Here's the only correct schematic I have found:
Link to the schematic in case the horizontal scroll bothers you:
http://oi63.tinypic.com/eqrqm1.jpg



With nothing plugged in, the preamp section is muted right before the tone stack, so turning gain and volume controls do not affect it.
If I ground the circuit right before the power amp (R22) the hiss is completely gone.
So, it must come from either the tone stack or those Sallen key filters.
If I bypass/jumper R15 and thereby disable that passive LP filter, the hiss is brighter but not louder so I think I can narrow it down to the tone stack or that first Sallen key filter.
Any suggestions on what to try next? Or any ideas on what could cause the hiss in that part of the circuit?

I have a Vox Pathfinder10 and you can honestly not tell if this amp is on or not when nothing is plugged in. Even with my ear against the grill cloth, one inch away from the speaker I still cannot tell if it's on. So it must be possible to have a dead quiet amp with nothing plugged in. And this amp is pretty bright as well and mutes/grounds the circuit in a similar spot.


Problem 2: OD channel volume knob mystery:
Another issue I can't seem to figure out is the following: on the OD channel, with the volume knob on 0 there is still some sound coming through. It's all treble frequencies, no bass.
When I ever so slightly turn the volume knob, like 1/10th of a mm, the bass frequencies kick in and the sound seems normal but it's actually already too loud by then.
I measured the resistance between wiper and ground to make sure there's no 'residual' resistance left when on '0' and it measures to about 1 Ohm.
One Ohm would give me 1/15000 of the maximum output (load of 10k + 5k pot). Maximum output of that opamp is about 2V because of the clipping leds in the NFB so output from the volume voltage divider would be 0.00013 volts. Can I hear that after the power amp amplified it? How would it explain the missing bass frequencies?

When you compare the circuit of a Tube Screamer, a Fuzz Face, a Rat and a Big Muff they look like coming from different planets, while if you compare the schematics of those cheap SS amps from the big names (Frontman, Pathfinder, MG, Crush etc.) they look like they came from the same house.

So, I wonder if those companies actually design these little practice amps each themselves or if they are designed by the same 'foreign entity'?

To counter some expected responses:
I do realize that I picked very specific pedals and that a lot of other pedals are very similar to each other.
I do realize that a honky, ratty or fuzzy pedal is for a 'niche target audience' while guitar amps are intended to suit as many players taste as possible.
I do realize there are only so many ways to design an amp, a tone stack, that all of them deal with the same guitar frequencies and all will want to produce as cheaply as possible resulting in a similar choice of values for resistors and caps and other components.
I also realize that both in pedals and amps, people/companies do borrow/steal ideas from each other.

Yet still, the similarities between a lot of those 'cheap mass produced big name ss amps' is striking. And so far, not a single one seems to stand out or do things differently enough to say: hah!.
They all use the same opamps. Fender isn't saying: we'll go with a ne5532 instead of the usual 4558 and our amp will be 1$ more expensive. It's not as if a Fender fan will buy a Marshall MG instead of a Frontman because it's 68$ instead of 69$.
Across those amps they all seem to be using the same few mechanisms for their 'dual channel' switching (see links and examples below)
Another thing that is 'suspect' is that the evolution/iterations of the models of these amp are the same across those brands:
At one time they used transistors, but when they switched to opamps, they all did so.
At one time all of them came with a real analog spring reverb (Frontman 15R, Lead 12R, Pathfinder 15R, crush 15R etc.) and when these (crappy) spring reverbs were left out in a new iteration of these amps, then all brands did so. Not a single one said: we'll keep it.
As far as I can tell they all come with speakers from the same manufacturer (Chunil speakers)

Obviously, they don't build these amps themselves but outsource it to some Far East factory.
So, how does this work? Anyone some inside info on this?
Does a Chinese 'designer' let Fender, Vox, Marshall etc. choose from modules (booster, clipper, tonestack etc)?
Do they get a list with available components and a rough total of components they can use and completely design it in house?
Do they get complete (prototype) designs which they can only fine tune and put their designer's name under it.
Or do they simply order a 'fender sounding' 'Vox sounding' amp and actually don't really care and have little to no input on the circuit design?

Here are some random examples of similarities between schematics where I'm thinking: what a coincidence...

Epiphone Studio 10 vs Vox Pathfinder 15:
http://archive.gibson.com/Files/schematics/Studio%2010.jpg
http://www.tdpri.com/media/vox-pathfinder-15r-schematic.27215/full?d=1349532940
Both using a pot to change feedback resistor of the first non inverting opamp and the input resistor of the second inverting opamp at the same time.
Clipping diodes always in the signal but '(de)activated' by a boost switch (operated through a FET) to '(de)activate' some some parallel resistance in the feedback of the second opamp.

Marshall valvestate 10 vs Vox Pathfinder 10
https://elektrotanya.com/PREVIEWS/63463243/23432455/marshall/marshall_valvestate10_10w_8001_8010.pdf_1.png
http://bencraven.com/wordpress/wp-content/uploads/2016/07/20160528_vox-pathfinder-10-schematic-1024x400.gif
The other way to boost and clip. This time diodes are switched into the signal while at the same time some resistance in the first non-inverting opamp is connected/shunted to ground.

Frontman 15G vs Marshall MG
http://blueguitar.org/new/schem/fender/frontman15g.gif
https://www.electrosmash.com/images/tech/mg10/marshall-mg10-schematic-parts-big.jpg
The third way: bypass the clipping section entirely and as coincidence would have it both use the same type of contour/tone/filter.

I'm not saying all these amps sound the same, far from it, but that a lot of them look like they were designed by the same person.
Maybe I'm looking too much into this or focusing on some peculiar but perfectly explainable similarities but I'd like to hear what people's thoughts on this are.

Are you 'naive' and wishful thinking like me and secretly hoping that even that Pathfinder you love so much was designed by someone at Vox that had Vox tones running through his veins and gave everything he got to capture the legendary signature sound of his company/brand in this little cheap ss amp?
Or do you think it breaths as much Vox as a Joyo ACtone: Chinese crap that happens to sound good but nothing to get emotionally attached to.

Also, why is there no Mesa 15G ? They seem to be the only 'big name' in this that does not take part in the 'cheap ss practice amp' field.

And as a last note: why do all of them, as far as I've seen, clip to ground (or the inverting pin which boils down to the same thing) and none seem to do the 'non-inverting feedback clipping'.
Clipping to ground, especially with higher supply voltages than the pedalwise 9V, does provide for a little more dynamics but at the same time especially with those small cheap speakers it gives fizzy, crackly note decay at edge of break up settings and 'bees in a can' for higher gain settings. This is what people associate ss pratice amps with and with feedback clipping you'd have none of that.