I have the Pass XA60.5 monos, absolutely love them, described them in enough messages on this board, thus won't bore you again.
As to electricity cost, letting them warm up for about 4 hours before using would get you 95% of their performance. The next level up would be 4 days, and that just does not seem worth the cost, needless to say being wasteful. But really, a 4-hour wait on the XA60.5 easily blows away most class A/B amplifiers out there. Now if you are in a rush, playing immediately after power on would already get you into the better end of class A/B amplifier selection. Nevertheless that's not the point when you purchase a class A amp.