Bel Canto Ref1000 MK2 Or Ref 500m ?

I read that the Reference mk2 series improved on the sonics of the Reference series.

The two amps use totally different ICE modules so you would expect they sound different. If you haven't alreadt done so, check here for info on the two modules. Plenty of specs for you to go through but IMO the newer ASX2 series modules used in the 500M have better specs.
BTW I am currently using the 500S amp in my system with good results. Although I have used several amps (300S and 300M) with the 200ASC module, I have not heard the Ref 1000 which uses the ASP series modules.

I have the Ref1000 mk2 and they sound phenomenal. Effortless dynamics, bottomless power reserves, articulate delineation of every note. Deep lows, smooth mids, and clean highs. Never lose composure in the most complex passages you can throw at them. Run cooler the louder you play them and idle at only 15 watts each. They take a LONG time to break in (I honestly think over 1000 hrs) but when they do, you get an incredible amp setup that can compete with much more expensive amps. I advise setting up dedicated outlets for each. I am running 20 amp circuits for each with 10 gauge 30 amp copper wiring with no power conditioner (I have surge at the main). These babies sound better than ever now (with 50 amp peak draw, they need some serious power supply). Who says you can't have too many watts ;-)