It can be an issue, need at least 2" of clearance for a rack install, I had to move my rack a few feet down the wall as it was warming the wall behind the rack and effecting the thermostat.. So to answer your question, warm with adequate ventilation. I also switched to metal top cover, as the 6550 tube distorted the acrylic one that came with it.
Very good amp, even better with real 6H30 "DR" military grade tubes from Bat Audio.
Later tubes including in my opinion ARC Factory tubes and Electro Harmonix PI Gold pin,
leave a lot to be desired. The difference was like moving from the REF 5SE to the REF 6.
Best of Luck,
John
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