I just came across this thread while browsing. Coincidentally, I started vertical biamping my Sony SS M3's with two Rotel RB-980BX's (2x120) just about a month and half ago. Previously, I had been using just one of these. Removing the binding posts and adding the second amp made a huge difference. Night and day. After reading this thread, I ran over and switched them from stereo to bridged mode. The volume jumped dramatically, inversely proportional to the quality change. Ick. The sound was very thin, almost fuzzy. Maybe I need to wire them differently? I'm looking forward to an active crossover. I'm sure this will yield significant improvements.
By the way, the preamp is a Lexicon DC-1 with "Bypass" engaged, no bass/treble adjustment, no Loundness Contour, etc. My source is a basic Sony Bluray player connected to the DC-1 by toslink. I lucked into the Lexicon and Rotels (both needed repair). The speaker purchase was a craigslist shot in the dark that I feel turned out very well. :)