My experience aligns with Eric's.... All class D amps I have had in my system require warm up.... Particularly older generation class D could sound rather dry at power up time.... To sound their best, 24 hours or more were a must.... More recent amps, such as Rowland M925 and M535 sound pretty good a few minutes after power up, but if you want them to give you their very best, warm up is definitely necessary.
Don't want to warm them up? No problem... Do what I do.... Leave them on at idele 24/7.... Being some 95% to 97% efficient, you will never notice their existance on your monthly AC bill, nor will they ever heat up your house..... Just remember to unplug your entire system when thunderstorms are around.