Timrhu's right: I turn the amp off for late-night listening. My pattern is to power it down, and then back up in the a.m. It's clear some think that's a bad idea, performance-wise. I can't say that I notice much of a difference, but it's usually been on for an hour or two before I do any listening.
I could turn it off only when using headphones, but figured that since I'm turning it off, I might as well leave it off for the night.