I didn't get my answers properly highlighted so I'm giving it another shot as well as fixing a couple of mistakes from my original post:
What Axiom subwoofer(s) do you own?
Where do you have the Level, Frequency, Phase, and Subsonic Filter (if applicable) controls set?
Level 7/10, Frequency about 120 Hz, Phase Normal - This sub has no Subsonic Filter and likely produces no significant output below 20 Hz
How are you connecting your signal source to the subwoofer (RCA/XLR/High Level)?
What receiver/processor/pre-amp/integrated amplifier are you using?
What are you using for your main speakers?
LFRs and VP180
Where do you have the crossover frequency and trim levels set in your processor/receiver for your main speakers and your Axiom subwoofer?
THX (80 Hz) but with fronts (L/C/R) set to Large
Are you using the trigger on your subwoofer?
Yes it has an automatic trigger that turns it on when a signal is detected on the RCA. This is set at "Low"
Just a quick update on this: I've been tweaking my setup to get the best results with the LFR's now in my system (no DSP upgrade yet). I tried a somewhat unconventional approach to help my not so great sub extend the already low range of the LFR's using the killer LFE track on the "Master and Commander" DVD which has good content down to well below 20 Hz.
I realized that setting the sub's low pass filter as low as it can go - labelled 40 Hz - might help to focus all its amp power on the really low stuff. This way I can also bump up the volume without impacting music content.
This seems to have worked as with the sub on I can definitely hear and feel some low rumbles in the cannon battle scenes that aren't there without the sub (and didn't seem noticeable before my changes).
Now I may need to send my DSP back for the upgrade...