Mesa Boogie V-Twin problem

[dukelion]

I hope it's ok to post this here.
I have a Mesa Boogie V-Twin pedal that's acting up.
Sometimes the volume drops very low, but the sound stays the same, you just have to turn the volume knob on the pedal way up to hear anything (though it's still very quiet).

Here's the schemo:
http://www.tubefreak.com/V-Twinp.gif
I can post pictures of the circuit board if that would be of any help.

I'm sure it would be very easy to fix if I just knew what the problem was.
Any help would be much appreciated.

[Fender3D]

Q7 is used to switch the volume off when switching channels or sound.
You might look at it, you might pull it off to check, the pedal will work the same, maybe some noises when switching...
Also, it might depend within the bypass section.
...Or any bad solder...