I use the MX-100 with just 2 mics (CM300s) all the time and have been very happy w/ the results. Yes, the center or blend channel is turned all the way down, but I havent been able to pick out any noise. (not that its not there, I just dont hear it). I think, for what its worth, the MX-100 is a great pre/mixer. Especially used w/ the CM300's.
also, my $75 guess was just that, and I should have mentioned that that was for a used box.