I own them for 4 years now. You really need an expert forn keeping these alive. They are like formula 1 racing cars. Patience and know how are key here. The most important caveat for having less trouble is a set of 8 6C33C tubes that are closely matched. All 8 need to have the same mA value! We decided to test the tubes our self.
Also the 6922 (2x) need to be closely matched. If all is well, the sound from this amp is breathtaking. They even drive my Avalon Ascent 5.5 ohm load wih ease. Volume is earsplitting. The sound stage is as big as the recording permits. Bass is tight. Dynamic swings are amazing, a dream come true.