Thanks for all the tips folks, I actually found the problem and was returning to post the answer. The IEC electrical cord socket mounts directly to the PCB on this amp (actually a small PCB dedicated to the mains switch and such). The board mounted IEC receptical must have been knocked around some, no doubt from strain on the end of the electrical cord plugged into the amp while the amp was transported and gigged for years. One of the solders or traces lifted on the IEC socket and blew a trace right there at the socket. I couldn't see it until I started removing all of the boards. J M Fahey's suggestion of tracing out the path starting from the electrical cord pin would have worked perfect to diagnose this.

The speakers are in good working order and work fine with other amps...

The power supply has a lot of capacitance on it so I wouldn't suspect that, especially if you replaced the caps already. Does it make the sound when plugged into other outlets in other hoiuses or buildings? If not, it could be your noisey power at your house. If not, it could be a ground loop hum if it sounds like a 60 cycle hum. Try re-routing those power wires along the chasis edge and twist the cables. Do the same for other wires re-routing as appropriate. Try adding some sheilding to the cabinet or between the trannies and other components. If that still doesn't fix it you may be getting some high or low frequency oscillation (possibly outside of human hearing range) that can affect your sound.

I have a Behringer PMX2000 PA head that won't power up when the power switch is turned on. It was working fine until turned on one day and it made a loud "POP" through the speakers and everything went dark.  All fuses are ok. Power cord, power switch, power supply caps, and thermal protection all test good. No obvious burnt traces or comonents. Any ideas? 

Also, Behinger doesn't supply schematics or allow them to be posted on the Internet. Anyone with a schematic for the power section of this amp please feel free to email me the schematic anonymously at dgushue(at)att(dot)net. It would really help me in testing voltages at various points. Thanks.