Legairre, on your opening question, although a 90Hz crossover might be more nearly ideal for QS8s, since the crossover isn't a sharp cutoff no "hole" is really created with the 80Hz setting and several owners use it with satisfaction.
As to using a single QS8 for a back surround, note that although there's no such thing as a 7.1 format(at most there's matrixed or discrete 6th channel material from DD and DTS), Dolby, THX, etc. recommend that the mono 6th channel material be reproduced with two separated rear speakers. The primary reason is a psychoacoustic reversal effect in which a sound coming from one speaker directly behind a listener will instead appear to come from directly in front of him. It's been suggested that with a speaker such as the QS which has a very wide dispersion this effect is less likely to occur. A second possible basis for using two rear surrounds is that when 2-6 channel material is processed with DPLIIx, Logic 7, etc. into seven speaker material the 6th and 7th speakers often receive different information and they should be separated widely enough so that listeners get both a left rear and right rear effect.
