Go for 7.1. I think bi-amping is snake oil, honestly. Certainly it won't provide any benefit with mass-market receivers such as the 905 (or my Yamaha 3800 which also offers it, with the same downside).
If you really want to bi-amp, run analogue outs to a pair of high-quality stereo amps that will cost as much as a decent receiver themselves.... or just spend the money on a good subwoofer and forget about it.
