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.
Problem with Bryston 14bsst
I seem to have an anomoly of sorts. I have a Bryston 14bsst paired with my deHavilland tube preamp. The music is perfect. I run a Wadia 302 through the pre with very nice results. The other day I tried an old Raytheon 6sn7gt tube in my preamp. I immediately heard loud hissing etc. from my right speaker but before I could shut down the system, the thermal breaker shut down the right output on the Bryston. It was extremely hot. I of course removed the tube and put my old 6sn7 back in the pre. Everything seemed fine for a few days. Yesterday, I turned on the preamp to let the tubes warm up for about a half hour. I got distracted and forgot to turn on the Bryston too. When I did turn on the Bryston it again got extremely hot on the same side as previously. I shut it down. I tried the system again later and everything seemed alright...however now it is doing it again. The Bryston heats up immediately upon the introduction of the preamp. I tried the preamp on the cd output selector and the tuner output selector...same thing...lots of heat on one channel. Here is the kicker. Just for the heck of it, I ran my Wadia directly in to the amp. The amp stays cool and everything is working! Looking at the history of when this started (bad tube) could I have done something to the output of my preamp that is sending a nasty signal of some kind? I look forward to any help as I am quite upset. I dread sending my Bryston across the country...or the preamp for that matter. Or what? thanks, Jim