How hot do a pair of Pass Labs X600 amps run ?

That idle figure gives about a 150 watts of class A continuously. I have a pair running at this moment as I write. They are breathing sublime life into the toughest speaker load known to audio. The blue back lit dial is resting quietly in it's customary ten o'clock idle setting while the music flows at 70 to 80 hertz, my comfort range.

600 watts converted to heat would produce just over 2000 BTU/hr.

Don't know about the conversion, but surely not all of the energy is dissipated as heat. However, if they are Class A 150 wpc, I would imagine they run pretty hot, unless the heat sinking is very extensive.