Small is probably the way to go for the Snells, but I couldn't find any specs online for the frequency response on these speakers. If you or anybody else knows what the -3dB point is for these speakers, we can make some good educated guesses for crossover points to start with on the fronts, center, and side surrounds.

There are a number of ways to set up those three subs. The easiest way is to split one of the 990's two unbalanced subwoofer outputs to feed the front subs and use the other unbalanced sub output to feed the back sub, and this is going to be a pretty decent approach. All three subs will get the LFE channel from 5.1 sources as well as low frequency material crossed over from the mains.