hi unreal, after reading what you noticed for the two types of supply caps, i wanted to tell you that i have worked on a few amps over the years that sound to be simillar in design. i could be wrong" not being able to see your amp for myself " but, it sounds like the irf640/9640 mosfets are your output stage requiring the 63 volt caps, but i think the 35 volt caps are for a pre-driver stage. if this is so, then like many types of "car" mosfet amps that have this same type of set up, the tl074 op amps have a pair of zenier diodes for the 12-18+/- volts for thier supply. so you might be having a problem in the pre-driver stage.. i hope this gives you a better idea about your set up.😀