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.