I was a McCormack dealer for many years and have owned most of the amps that Steve has made. Excellent build quality and sound. Couldn't go wrong.
IMHO, Belles is one of the most underappreciated and unknown high end manufacturers. I have owned alot of David's gear and find it top notch. Just SUPERB and deserves to be held amongst the best out there.
The Belles gear tends to the yin side of things. Warm, open, a little dark and romantic, (my kind of equipment). The McCormack is the yang. Fast, bright, clear, detailed.
If your system is already a little bright, the Belles would be the way to go. But if your system is a little dark and slow, the McCormack would be better.
I have owned literally hundreds of amps and the Belles is one of my favorites. In fact, once I can afford it, (I'm going back to school), I plan on buying a Belles 350A Reference or a pair of MB-200s or a MB-500 and calling it quits. My last amp(s). I'm just smitten with the Belles stuff.