I know from Home Theater (watched the Blue Ray of the Rock and Roll Hall of fame 25th Anniversary last night) that I sized amps very conservatively. I use Sonus Faber speakers, which are not super efficient but give good midrange. They were never really happy until they got 200 WRMS of Pararsound each. Biggest improvement of size is in bass control, biggest improvement of quality is in high end fielity and consistency.
I have been nervous about the Class D amps since they haven't been well received in Absolute Sound and other pubs. But they do have some virtues for car audio, including size and heat.
You are right that receivers are voiced differently. Its a no brainer to hear the differences when you audition the same system and swap in different receivers. Adjust gain to match and you will hear differences. I only use my denon for the front end pre-pro. Even the larger denons did not have the sound I wanted.