I never managed to hear the bel canto 500Ms (not for lack of trying on the sparse used market)--but I did find a cherry pair of 600Ms. The main difference between them is that the 500Ms feature an older class D amp module, ICEPower vs the 600Ms using NCore.
More important the class D module used is the input stage and overall design & construction. I can only say good things about my 600Ms. They have a beautifully clear, but not at all sterile or edgy sound. They’re easily the best SS amps I’ve ever heard.
But ICEPower can be very nice, too. My first class d amp was the Wyred 4 Sound ST-500, a stereo amp based on an ICEPower module. It has a wonderfully musical sound. I kept it as a backup; it was so good to me for 3-4 years.
I’m guessing the bc 500Ms & W4S ST-500 would be roughtly equal in how hard or reasy they are to find used. You can’t go wrong with either IMHO.