I wish you would reconsider using only the rear speakers with multi-channel music. Our sense of acoustical space comes from lateral (sideways moving) sound. Sounds from other directions (above, behind, etc) add to envelopment, that too only for frequencies above 1 to 2 kHz. However, sounds from these directions don't excite the lateral cues needed to convey spacial information.

The best and easiest way to reproduce lateral cues is with speakers at your sides. These are also the locations that helps most with low frequency envelopment (if you're into stereo bass and all that). Adding rear speakers will give even better envelopment and excitement in the surround field.

BTW, you don't need fancy processing if you're going to use both sets of surrounds; simply copy the surround channels from the sides to the rears. Hopefully the 990 will allow you to set time-alignment individually for side and rear speakers; the quadruple crossover will allow you to have larger side speakers and smaller rear speakers.

Finally, you can do a simple experiment at home to hear the effects I've described and confirm the results for yourself. First, unplug your side speakers and listen to some multi-channel music (or 2-channel music via PLII). If you raise the volume of the rear speakers, you'll notice the front soundstage becomes louder]. Next, listen to the same music with the rears off and the sides on. As you raise the volume of the side speakers, you'll notice that the front soundstage becomes wider.

Best,
Sanjay