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

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.

Ah! and thus the 200 fully class A watts XA housed in the formidable X - 1000 chasis. This should give pause to owners of the trim mega watt "class A" amps that abound.