First, this is a guitar amp, not precision laboratory gear, so close is close enough. If your 15v rails are within half a volt, I'd be happy as a clam. In fact it matters less to me if they are 14v, 15v, 16v, than it does that whatever it is, the positive and negative are pretty similar in voltage.
The 42v or whatever, same deal, just ball park is good. So you have them.
First we look for loose connections, so ball up your fist and whack the top of the amp sharply. Does this affect the sound in ANY way? Does it make the volume return, or does it crackle when you whack it? Turn down any reverb first.
Turn the FX loop balance control back and forth a few times, any change? Plug a cord from FX send to FX return, any change?
Plug the guitar into the FX return to bypass the preamp, does the sound still have the problem or is it fairly clean and strong? Likewise, run a cord from FX send to some other amp, how does it sound over there?