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.

Sugarbrie ... I don't think warmup was the reason. Warmup may make something of a difference bu nothing as profound as what I heard. I think fatparrot is close to my experience. You may not notice the step up, but boy do you ever notice the step down. This applies to many other areas of life too.