I have a pair of Audio Mirror monoblocks that use the 6C33C tube and have more than 50 hours and no problems at all. I like the sound of these tubes - very clear, uncolored, balanced and almost no real sonic signature. Tubes with other amps I had always seemed to accentuate some part of the music - EL34s seemed slightly loose in the bass, but great for voices, KT120s were zippy on top, etc. etc. The 6C33Cs are auto-biased according to the Audio Mirror website. Maybe that is why I have had no tube issues yet. But I do have a backup quad! LOL.