You knew the volume pot worked because you could hear it change things throughout its range.

Determine whether your hum is 60Hz or 120Hz.   Or if not in USA, then 50Hz and 100Hz.

Turn it all the way down and solve that hum first.  I suspect you have two hums going on.  You have some sort of hum coming in from the early stages, and it is out of phase with whatever hum is after the volume control.  SO at some middle setting of that control the two cancel.  That explains your 2 o'clock thing.

A schematic would be nice.

Just noticed a drawing error.  The two output transistors are drawn wrong, they have swapped base and collector connections.

Quite possible one of those two parts is bad.

I used to rebuild Seeburg Juke Boxes.  And in the tube era, there would be two large chassis.  One was the amplifier, the other the control center.  The control center used tubes too.  A 2050 thyristor tube acting like a triac to fire a solenoid, 12AX7s amplifying pulses from the core memory, and other circuits.  Some of those tube circuits ran plates at ground and a -300v supply into the cathode circuits.

OK, that tells me the output stage cannot provide the current the load demands.

You have 8v instead of 7.5 on the supplies, well close enough, it is just a guitar amp, not lab equipment.

verify the two 0.5 ohm 5w resistors associated with the output transistors.  And for that matter, the 150 ohm and 22 ohm resistors associated with the drivers.

From the speaker on the schematic, follow the output line to the left and you come to 10 ohm R124.  Is it open or burnt?

g1, see my post two above your earlier one, is my explanation ffaulty?  I don't think the drawing is wrong.

Perfectly normal for the two 330 ohm resistors to get hot.  Hopefully there is about 7.5v across each of the two diodes.

You say it sounds OK through headphones, but does it still sound OK through the phones with the speaker also connected?  Oh wait, you say the phones makes the speaker silent?  The schematic does not show a cutout, but we can deal.  With speaker connected, how does the line out sound?  Run that to some other amp.  Does the line out sound change when speaker is connected or removed.

You soldered some stuff and it came to life then failed again.  SOunds like you have a bad connection or cracked copper trace somewhere.  Not so much a bad part.  Transistors don't generally die and heal themselves.

Look at the data sheet for the 3101, note it runs on -16v, that is a negative supply.  SO if they connect +15 to ground and ground to the VDD terminal, the IC sees a negative supply.   The VDD terminal is 15v more negative than the +15 supply.

Your MN chips are probably fine.   Remember the chorus circuit path is a lot more than the MN chips.  Certainly U6, and Q6 could be leaky as well.

Sounds like someone already uploaded a file with the same name as you want to use.  Try renaming your file first., or resave your file under a new name and upload that.

BLows fuses?  Start at the start.  DISCONNECT any speaker, if it still blows fuses, then check for shorted output transistors.

Guitar speakers naturally roll off the highs, and I wouldn't think of them for piano, at least not as a first choice.   I would think a search of PA speakers might do better.  And if you are using a guitar AMP to drive the speaker, then double no.  A small PA system, and so many now include EQ, which provides any mid-scoop you might want.

And the factory doesn't need to be there, ANY competent amp tech can repair this.  I bet you have a local shop.

I surely wouldn't assume the same zener needs replacement.  It MIGHT of course, but frankly it is more likely your FX loop jacks have dirty contacts.

Don't read too much into it.  A 4 ohm load on an 8 ohm rated amp won't automatically blow it up.  The poor thing will run out of gas if you push it, but polite play won't hurt.

Also, the load impedance will have no effect on the filter caps.

The compressor is basically just a compressor, the control is not a compression level or "amount" control.  The compressor control is the threshold.

Refresh the connections to the DFX card.

Schematic please.

I could be way off base, so consider that.  But you have both 15v rails at the zeners, but do you get both rails right at the IC, IC8b?

Just thinking, but normally that IC sits between the two 15v rails and centers between them.  Well, the center between plus and minus 15 is zero.  And whatever appears coming out of that IC is what is going to output the amp.  At least at idle.

SO what if one of the power rails doesn't make it to the IC?  Then the poor thing tries to center between 15v and zero, which is 7.5v, and that is what is on the output.  Obscure, yes, but possible.