Biamping mono's worked the best for me, too--dual monos x 2. Also mono'd pre's with this system. I think if you mono the pre's you don't have the output impedence issue. I just used two identical stereo pre's but used one pre for each channel.
Of course, monoing everything can't mitigate poor SQ as Bvdiman points out.
Of course, monoing everything can't mitigate poor SQ as Bvdiman points out.