Mike, I don't know your room as well as you do, of course, but I'd have thought that 2' back from the listening position would have been just about perfect in a 5.1 setup(directly to the side for 7.1). With the wide dispersion of the QSs can't see using two sets of them fairly close together playing the same channel.
As Chris mentions, the DPLIIz can now use the front height speakers as an alternative to the back surrounds. Because of the ambient nature of the material(when some is in fact present)I'd think that QS4s just a few inches below the ceiling would be good for your room.
