mono amps one weaker than the other ????

i have a pair of high end mono amps from deja vu and after about 2 hours the soundstage moves distinctly to 1 side, i tried switching cables, sources etc... but nothing else at this point, question could a weak tube work good for a couple hours then show a decided loss of output or does this sound like something more thanks in advance

have you switched the monos themselves? sounds like a very odd problem to me. you might wanna check with deja vu if you haven't already.
i did call deja vu and they indicated a driver tube more than likely the issue, i hooked the amps back up as they were and the issue was still present,so i pulled the drivers cleaned the pins and switched them sound leveled out again and i have a good image again, still odd how a tube can play great for 2 hours then all the sudden loose contact or what ever and play at half volume

Switch the tubes between the amps, and see if the problem follows. If it does(assuming you have the Conquests): switch the pair of 6SN7s, and see if it follows again. If not try the 5U4Gs(A rectifier going south would certainly lower your output levels). Of course, it could be an output tube.
switching the tubes and cleaning the pins did the trick.....amps sound wonderful now thanks

I ran into the same problem with Vu's 300b push pull amps...required rebias of both amps at his lab. that was 9 years ago. Amps still going strong...
I'm having the same problem with my new system. My problem is in my pre amp and started when using the phono stage it took a zap or something. I have the c500T from mcintosh. Don't know what the problem is but it comes and goes. Plays fine then the stage moves to the left and the meters on pre are not even and the amp reflects less power on meters too but it starts with the pre amp meter on the right being lower by about 2db. They are going to send me another one but man wish I knew what the problem is.