Although not mentioned, is your amp covered with a cage? I had an amp where the optional cover had poor side venting. The tubes did not last even when biased conservatively. I tossed the cover and the tubes (EL-34s and formerly KT-88s) are lasting years. In other words, do not discount adequate ventilation.
Also, it you look at the tubes with the lights out, do the anodes (plates) glow orange at all? If yes, a clear sign of overbiasing.
Also, it you look at the tubes with the lights out, do the anodes (plates) glow orange at all? If yes, a clear sign of overbiasing.