Tubed Amp Troubleshooting

Need a bit of help/advice from the tube guru's. The mid/bass on my right channel started to drop out upon warm up this weekend. System will sound OK for the first hour or so after amps are turned on, but the mids/bass will drop out suddenly. At first I though it wast the mid/bass driver on the speaker, so I switched the speakers. However, the problem is still there with the speaker that worked fine on the left channel before. Last night I put in a new 6AN8 driver tube, but the problem cropped up again. Next step is to put in a new quad of power tubes and see if this fixes the problem. I will try that tonight, but wanted to see if anyone had other troubleshooting ideas. Thanks!
It seems odd that the amp would have a drop out in a specific frequency range since it just takes the signal and amplifies it. Your bad tube guess would suggest you don't have the power to drive the bass. I would expect this to occurr from the get go since, if anything, tubes will sound fuller as they warm up. Did you sequentially switch the source and pre-amp to make sure the problem stayed with the amp/channel in question and isn't in another componant? sorry i can't help more
First thing I tried...but I'll try it again.
i hate to sound like an idiot but maybe you have a cable shorting.
Well, it seems like it was the power tubes. Anew quad of KT 88's seems to have taken care of it. For further referecne, the problem was present with both analog and digital sources (each going into different input of the preamp via different cables). Switching the left and right channels at the preamp output showed the problem followed the amp (and associated cabling) on the right. So, this told me it wasn't a source component or preamp issue. A cable short is not totally ruled out yet, and will be the next thing to check if the problem comes back, but for now it seems to be ok.
I am not sure how many tubes in your amp. Last time I have trouble with my tubes, I swapped the tubes from left to right to make sure they are the source of the problem. Depending on what type of tubes you are using and how frequent you use them; it is alway a good idea to replace them after a one to two years. For me, I have 4 sets of different brand of EL34's for my amp. Each have different charateristics.
Well, so far I've had no more problems (after putting in new power tubes). The amps are Aronov 9100, each with 4 KT 88 tubes. The old tubes were about 18 months old, but had quie a few hours on them since I've burned in 2 pairs of speaker cables and three sets of power cords with them since I bought them