I have an X350 that I leave on continuously. It's roughly the same as one x600 monoblock. The top is slightly warm but the heat sinks are usually around the 'mug' temperature of a 3-5 minute old cup* of coffee.
I would say that it generates
A) Less heat than my ARC VT-100, old Aleph 2's, and likely
my BAT VK-5i tube preamp.
B) More than my old 185 W McCormack DNA-1, 200w Ayre V-1x,
and all 100-200w non-aleph amps I have used.
C) Around the same as my 4 KT88 tube AES 25 tube amp, and
possibly my 2 300B tube Cary 300-SEI
* The cup is glass, 3mm thick, green. My employer's stunning interpretation of an inspirational and appropriate substitute for employee pay increases. Rather pretty.
** Actually one of a set. The other being utilized as a spit cup in the upstairs bathroom.
*** Now that I think about it, I am strongly considering migrating the first away from coffee duty, and into a new role as a receptacle holder for decimated shellfish casings. Opinions ?