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

Think about a block of metal that's 19x12x22 inches that is hot to the touch but not so hot that you couldn't keep your hand on it. This is in your room radiating heat. For a typical listening room, this isn't terrible. Then there would be two for stereo. Another way to think about it would be to imagine a lightbulb array of comparable wattage -- the great majority of a light bulb's wattage is radiated in heat, not illumination. 600 watts of heat from an X600 would be like six 100 watt light bulbs. (Or one 150 watt light bulb, per the wattage cited in the above posts.) Notably, the heat radiated might actually decline very slightly as you turn up the volume, so long as you stay in class A mode, since more of the power draw would be consumed in making sound. Of course, it might get a teensy bit hotter when you start peaking into class AB. The X600 is a fine amp. Good luck.

to heat.

Conservation of energy. The energy is always there, though the form may change.

Draw 600 watts of electricity in an amp and 100% will be converted to heat. Even if some is used to produce sound, this sound will degenerate into heat as it is absorbed or otherwise dissipated.

What's your electric bill like running these babies? and without?...

In a house with 62 light bulbs, all of which are left on too much due to kids, the initial extra cost of the monos was absorbed without raising WAF ire, and that is a good thing.

All of the 600 watts are lost to heat when not running. I suppose, at peak 1800 watts, that too eventually winds up as heat. The speakers are huge radiators.

The heat sink temperature on my 600's is about 114 degrees Fahrenheit at idle. The amps contain a thermal cutout that will shut them down if the heatsink temperature exceeds 167 degrees Fahrenheit. This is considerably cooler than the heat sinks on my old Aleph 1.2 amps.