Mitch,

The crossover point for Marantz equipment is 80hz, not 100hz. What is the frequency response of your "large" main speakers. Just because they are large doesn't mean they have great bass extension. The midrange should not suffer at all when the bass is going to the subwoofer. The opposite should be true. Your main speakers won't have to strain to reproduce low bass and the midrange should clear. Yes, you can hook up via speaker level outputs but I wouldn't suggest this fashion. Unless your main speaker go EXTREMELY low, loud and strong I'd set the mains as small for all media.

Good Luck,

Bill Lucas