I can't see all of it very well but here's a rough schematic

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It is in post #3

Here is the Tremolo-Reverb board
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Here is the input schematic. Please post another photo of the tremolo-reverb board.
I can't see the circled area very well.
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Here is the schematic of the power amp board. I believe it is correct but there could be mistakes.
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Are there any numbers on those 4 transistors? The blue 1000uf cap seems to be reversed.You cannot view this attachment.

It's in this schematic: 130 ohms/1W

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SVR1 is to set FET bias for the compressor. Don't touch it. I don't know how to set it.

I guess Q4 is not the problem. I'm running out of ideas. Are you sure there is something wrong with this amp? It's normal for the overdrive channel to be louder.

If the overdrive channel sounds like it should with a guitar you don't need the check it with the oscilloscope.
Leave the amp switched to the clean channel and remove Q4. See if there is a difference with or without Q4

Try the amplifier with Q4 removed. The clean channel will be always on even when switched to overdrive but you can see if there is a difference in gain.
You can measure the resistors after desoldering one end.

If the overdrive channel is OK, the problem in the clean channel must be R44 100K, or Q4 2SK30A, or R41 10k
All ICs in your amp are dual op-amps like this

You need a stable tone of some kind. There are many oscillator apps for phones and computers or you could use a keyboard organ sound, just tape one key down.
Plug the tone generator into the input and check its level with the scope. Set the vertical gain for about 1 division amplitude on the scope, then move the probe to
IC6 pin 1. the amplitude should be about 5 divisions. Turn the Clean Gain full up, move the probe to the center terminal of the gain pot. The level should be the same 5 div.
Move the probe to IC6 pin 7, should be more than 5 divisions amplitude.

I posted this photo at mef a few years ago: https://music-electronics-forum.com/forum/amplification/guitar-amps/maintenance-troubleshooting-repair/44114-possible-to-repair-a-loose-potentiometer-leg/page2

I ground out these pliers to use for AC cord strain relief installing/removing. They work very well for that, so when I was looking for a crimper the tool was at hand. I have to be careful not to squeeze to hard. It can easily break the wafer.

Sometimes the terminals get loose. Check them after they're removed from the board. If the terminals can be moved side-to-side at all, crimp them or just replace the pot.
I had a Vox amp a while ago where almost all the pots needed to be crimped

C6 and C7 are part of the compressor in the overdrive channel.