Regarding the rear speakers, or "rear fill" as some people call it, I definitely like the added mid-bass the rear door mid-woofers provide.
With the rear speakers on the "LOW" channel on the DEX, I did experiment with cutting the high frequencies out by setting a 2K crossover point for a low pass filter, 18db slope.
To me, and this is probably a personal preference, with all the high frequencies coming from the front speakers it sounded almost too precise, to easy to tell the direction of everything, and made the stage much smaller. I tried this for a day or so. Then I set the read door slope to "PASS" and let the high freqs through once again. Everything opened up, the stage became much larger, wider and deeper, even deeper forward, the vocals became "angelic" again, especially the female vocals. Call it ambience, or whatever but I am one that prefers this setup. It sounds much better to my ears, and I mean much better. My system is slowly becomming an SQ hall and I'm lovin it!