So this is where things sit at the moment:
EQ consists of a boost at 100Hz, cut at 50Hz, plus the low end boost. Going with Nathan's earlier advice, I have a high pass filter in place at 20Hz, which is offset (and then some) by low shelf filters. This allows useful extension below 20Hz, while protecting the system from the strain involved with low single digit content. As is, there's quite a bit (it varies given the HPF, but up to 8dB) of ULF boost at play to produce that graph. I figure where it's at is a good stopping point, particularly given that I run the subs a little hot anyway.
As far as subjective thoughts go, the project of adding the 21.0LX and moving the 18.0 near field actually made more of a difference than I expected. While the basic sound quality of the system @ the money seat hasn't changed much IMHO (I felt I had the system dialed in well previously, so no shame in that), the power / feel of the system is significantly better. Going through the Blu-ray Reference Demo disc bass scenes, with goodies like War of the Worlds (Pods Emerge), Flight of the Phoenix (Crash scene), U571 (depth charging), was a completely different experience than what I've had previously, with my chair (and a half!) transmitting copious amounts of bass to my rear end. I also had some fun with the Skadoosh scene from KFP, gradually raising the levels while watching for compression on Omnimic's RTA. I gave up after hitting reference level plus another 5dB bump for the subs, while the 21.0LX/18.0 combo just kept on delivering more output. Since its a rare occasion for me to listen even at -5dB from reference, I think headroom to spare is an apt description of my new situation.