410 130 goes "bassy" after warming 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.

When I have one of these spooky things happen, I ALWAYS look at the caps first. It sounds like a cap gets warm and start going t!ts up. If you have swapped the op amps around and it doesn't go with that component, I'd start looking at the caps on the front end before those op amps that you have switched, then work your way towards the finals. The ONLY other thing I would look at FIRST are the resistors in front of those op amps that have any connection to ground. If they are going to sh!t when they get hot, they may be opening up. Its a thing that you almost need a scope to look at the signal before and after the breakup. Just my 2c worth. mike.