“actually, if your mains reach down into the 40Hz range you could experiment with running them full-range, and thus placing subs at a proper, minimum distance would add to the number of bass sources (mains + subs) for an effective DBA and smoother acoustical response.”
That is exactly what I do, upon the recommendation of the manufacturer, and the result sounds great. I also tried a fairly high-end, balanced high-pass filter, but I like the sound better with the mains run full out. I cross over to the subs at 45Hz. The trick is to go easy on power to the subs. A little bit is probably just enough in many cases. My main amps provide a lot of juice, which probably helps reduce any negative effects of having the amps driving the main speakers through the full frequency range.