Steve,
Alan has posted several pieces of info regarding the advantages of the quad-polar design for rear channels (i.e. QS4 and QS8) and that Dolby labs actually recommend the "rear" channels be placed more to the sides in the back than the actual rear of the listening area.
Try going to the "main index" and search for some keywords that seem logical to your question and see what you can find.
My personal opinion, based on your post, is that you should go with the QS4s. I don't know you LR layout - but I think you would have the easiest time finding a suitable location for the high dispersion design of the QS4s. The timbre match would be (for the most part) perfect.
As for music - most people (like you - and me) prefer to listen to music in the normal 2 channel mode. If you choose to use a matrix decoding/ambience (Dolby PL, etc.) effect so that sound does come from the rear, it is usually extracted to give you a sense of depth (i.e. artifical room size) in the form of reverb and such - and in this case the overall frequency response of the speaker (i.e. lack of bass) is not as critical as the front channels.
If you don't have a powered sub, I highly recommend you get one to (really) complete your system.
Good Luck,
Randyman
