Here is the schematic

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The 100K resistor along with the resistor from the input jack sets the input impedance.
If anything I might make it larger like 220K or 330K. It does not change the gain.
Gain of the first stage is set by the feedback resistor divided by the resistor from
the - input to common.
In this case common is the +12V. The gain of the first stage is 1+47K/2.2K = 22.4
You could change the 100K to 1M but you would want a FET op-amp  like TL071 for the first stage

Here is what I get with my pencil-and-paper style

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The 5534 often needs a small compensation capacitor between pins 5 and 8, usually 22pF.
Without that it might be oscillating thus drawing more current.

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