Best way to compare amps?

If you have some extra speakers and cables, then leave the other amp turned on in another room, so it is warm when you make the switch. A cold amp is going to give you a bad first impression against the other one. Many amps take a couple hours to get up to speed from a cold start.

I am guessing that part of the reason Sean above did not think the Mission was better at first is because of a cold amp (and switching back to the NAD).

You maybe can just leave the other amp on with nothing connected to it, but I am never sure if this is good for the amp.

What Fatparrot is saying was my experience with power conditioners and speaker cables. May help explain why some claim they make no difference at all. They plugged in the conditioner or new cables, heard no big immediate difference and gave up. I also had the same experience with my REL sub. When I shut is off after having it for a while, I thought: "Where did the music go?"

I was just guessing Sean. Comparing warm components is extra important if you are not making an upgrade, but are trying to choose between two equal amps and you do not have the luxury of weeks to get a feel for them. They both sound great, but which one do you buy? I try to avoid going to an audio dealer when they first open. Everything is cold, including my brain. I also usually go back more than once before getting out the plastic.