Best way to compare amps?

A couple of days would be better than a short period. If you can swing it try a couple of days each. At minimum a couple of hours each.

Sometimes you have to switch back and forth to appreciate it. I remember when I changed my NAD 3020 for a Mission Cyrus 2. At first the Mission amp didn't seem so much better. After a day I put the NAD back and whoa .... what a difference in dynamics, soundstage. The Mission amp went straight back in and the NAD was sold the next week.

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.

I would suggest listen to them for a couple of weeks ea. minimum because you have to both let it run in with your system & also audition under varying circumstances which include different times of the day. Also you might want to experiment with/without power conditioners & PC's if you have them. Good luck & let us know your results!