The MA6900 has autoformers while the MA6500 does not. Some people swear by the Mac SS amps with autoformers as the Holy Grail. But I've read that direct comparisons between these two hasn't reflected much of a difference. I guess it all depends on your budget and whether you can listen to both. It also might depend on your speakers and whether you're really going to push them hard. That said, I've read that they are both excellent.