Originally posted by gammonall:
The question is to what degree does the quality of the rears influence the sound in a 7.1 setup when they would seem to be used for fill in only.
When listening to music in surround, the rears add envelopment (mostly to frequencies above 1kHz to 2kHz). You can hear this effect for yourself. Raising the volume of the rear speakers will make the front soundstage appear louder, bringing it into the room. (Interestingly, if you raise the volume of the side speakers, the front soundstage will appear wider.)
With movies, the rears speakers can often play back as much sound as the side speakers. A good chunk of correlated content in the surround channels is cancelled from the side speakers and sent to the rear speakers (which is where it would have phantom imaged anyway).
To answer your question, I wouldn't skimp on the rear speakers any more than I would for the side speakers. Both surrounds pairs have to blend in with the front soundstage (the sides moreso than the rears). The one place I would save money is on bass response: get the smallest rear speakers that match the sound of the rest of your speakers.
Thoughts on a bi-pole set up for the rears?
If you're going to use PLII
x to create a 7.1-channel soundfield, then I would use monopoles for the rear speakers. PLII
x extracts two surround-back channels, and bipoles could blur the intended stereo-rear effect. As it is our human hearing is not so hot behind our heads, so it helps to have monopoles back there in order to better hear the left-vs-right imaging.