It really is not about "loud". It's about emotionally impactful. You want to be able to hear and feel pops and smacks and position sounds in a wide, deep, tall soundstage without sensing any speakers whatsoever. The M5s were a huge upgrade over my M80v2 in this regard. With the 800v4 over the 600v2, it was pretty awesome.
I'm using a 1500-2 to power the actives' HP drivers and a 1000-8 to power the mids and highs. I think it may be beneficial to move to a 1500-5 for each channel for an improved noise floor. I'm not sure how much practical difference it would make.
