OTL amps, my pick for the tube used.

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.

I am lucky that my listening room has 2 vents in it for cooling and temperature is easily controlled by opening/closing the vents. Very cool in summer as I live in SC and it gets very hot here,  but heat from the amp is a non-issue as we have to run AC anyway. The amp gets hot but the heat sinks help plus I put Isoblocks under the footers to raise amps for circulation underneath. One vent across the room blows directly towards the amp. Just lucky, as my last amp was in same location. I really like this amp and think I am done with the search.