Without a scope, tuning these problems down could be a bitch. Have you cleaned the effects send/return jacks and pre in/out jacks? Have you run a variable preamp level signal into the main amp in or effects return jack to isolate the problem to the preamp or the power amp sections. Just off of the top of my head I would be suspecting 16 year old capacitors in the various power supplies. I would start by checking the ripple voltage and symmetry on all of the power supply rails. Again. Much easier to do with an o'scope. But a meter can tell you a lot too.