HELP - sound goes "bassy" after warm up

WE HAVE REPLACED THE FILTER CAPS � ALL 9 OF THEM. It�s not the filter caps. On the main channel (reverb/trem) my 410-130 will sound great for about 20 seconds then the tone goes totally bassy. If I hit the strings really hard it snaps back to its regular brighter tone but it will only last for a few seconds. The other channel works fine all of the time.
I have swapped positions of all the op amps (and returned them to their original places, tried it with the bright switch and the deep switch on or off, unplugged the reverb tank, tried it with a different MM footswitch, tried both input 1 and 2 and generally prodded around the circuit board to see if the board was flexing or there were any cold solder joints. My electronics/tube amp friend has taken a pretty good look at it and helped me replace the filter caps. This chassis is immaculately clean and has never been stored in any adverse conditions. We simply can�t find anything obviously wrong with it.
Anyone else had this happen? Did you get it diagnosed? Any help is appreciated, thanks.

The fact that one channel works all the time means that it OBVIOUSLY has to be in that channels preamp. I think it probably has a cap gone bad in the preamp. personally, since those amps are a pain in the a$$ to get the boards loose and get underneath them, look at the schematic and just get every cap that's in there and replace them. It's probably got those yellow caps that were pretty cheap back then. I know, it's a shotgun approach but it WILL isolate the problem faster than taking each cap out or the circuit and checking for dc. I don't have the schem in front of me but little caps like that are cheap and it may go faster than one at a time diagnosis. There can't BE more than 6 or 7 in the one preamp chain.