It sounds like you have DC leakage in 1 channel of the pre amp causing the pwr. amp to get overly hot. Pre amp needs service.
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Well, the preamp could be doing what you guessed or the Bryston could be damaged (as you fear). Have you tried switching the left and right channels between the preamp and the Bryston? I.e., connect the left output of the preamp to the right channel of the Bryston and vice versa. If the Bryston overheats on the opposite (left) side, this would suggest the preamp has a problem. If the Bryston still overheats on the right side, it's likely the problem is in the Bryston.
Try swapping your left and right 6AH4 tubes to see if the problem changes to the left channel. I can't be certain, but I had a similar problem with my UV when a 6SN7 Tungsol blackplate went highly microphonic, and may have shorted. I believe this damaged the 6AH4s. Swapping in a new pair of 6AH4s brought things back to normal.
I would suggest a different approach. If one channel of your preamp has caused damage to one side of the Bryston, it will likely cause damage to the other side. Is this what you truly want to test? It's one thing to swap cables and channels if you have no sound, but if damage has been caused, why risk damaging the other side? Before doing anything else, put a meter on the output of the preamp channels and see if there is anything significantly different between them such as DC voltage, a much higher output level, etc. Also, put your cables on the meter and check for any differences as well....and flex the cables a bit while you do this.
So unless you are willing to take the risk of destroying the other amp channel, I would be weary of following the advice of the other posts.
I would like thank everyone for their help. I think I have it narrowed down finally. It turns out that their are poorly placed bias circuits on some of the Bryston 14bsst's which can cause "runaway" thermal events upon cold startup. I thought for sure that it was the preamp but after calling Bryston USA tech service they more or less said, "Yes, we know what that is about!" Evidently, there is a diode that was not properly secured on the heat sink of the 14bsst's. I have to send my 85 pound amp to Vermont to have four, 15 cent parts replaced and secured. By the way, I tried every one of respondee's suggestions before I finally narrowed it down. So if there are any Bryston 14sst's out there...beware of this thermal anomaly. Thanks, Jim