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.

C1 and C5 would affect the overall volume, so they are not the problem. It might be C26 or it could be something else.  If I had this amp I would plug a 500Hz oscillator in the input and trace with an oscilloscope or signal tracer which is basically a small amp and speaker with a probe connected to the input. The "shotgun" approach of guessing and replacing parts often leads to frustration,
at least in my experience. On the other hand you might be lucky.

If the only problem is that the clean channel is low volume, the only capacitor it could be is C26. Though the problem could be caused by something other than a
capacitor like a bad volume pot or maybe Q4.

I seem to have a lot of time on my hands lately. Here is the rest of the schematic

Here is an update of the preamp output ad the power amp schematics. In the power amp C1 and C5 could affect the gain. You will probably need to trace the signal to find the problem.
Q1 is for power-on muting. You could try removing it temporarily to see if that makes a difference.

How can i check the IC1? Red sonda on pin 1 of IC11 and black sonda on ground?  Yes

There are 3 electrolytic caps that could affect the reverb C31, C35 and C36 all 1uF. Try replacing them.

The reverb pot is after the output of the reverb unit.
If you hit the reverb unit do you hear the springs rattling through the speaker?
Unplug the 2 RCA plugs, set reverb pot and master on 10, touch the tips with your finger. One will cause hum in the speaker. That one goes to the OUT jack
Check the voltage on IC 11 pin 1. Should be 0V
Make sure nothing is plugged in the footswitch jack. If the ring connection is shorted to ground it will cut the reverb.

Connect a wire on the board like this. It will bypass the reverb pot.

The wire from CN2 IN on the preamp board is the reverb send. It plugs into IN on the reverb unit. OUT goes to CN3

Here is a rough schematic of the preamp output. Another possibility for the reverb is the input and output jacks might be reversed.

Plug a cord from Effects Send to Effects Return and see if it works. It could be the Effects Return jack switch.

Please post photos of the foil side of the 2 boards and another of the power amp board with the ribbon cable moved to the side.

Both the External Speaker and Phones jacks have switches that could cut out the speaker, also check the .1 ohm resistor R16.