Question about making 2.1 home theatre work

I've been using a Panasonic XR45 receiver for 2-channel HT for a few years. I tell it "no center, no surrounds, no sub" and it mixes down to 2 channels and sounds very good taking digital coax from DVD and toslink from TiVo.

Now...I want to put a REL sub into the system. The great thing about REL subs is that you can use them simultaneously for .1/LFE and to augment bass from your other speakers. But it appears that once I tell the XR45 that I have a sub, it insists on sending everything from 100 Hz down to the REL, whether it is LFE or not, and probably filtering the signal to the front speakers. That's not what I want to do. I want to continue to run the front speakers full range (I may eventually augment them off the speaker taps using the REL Speakon connection) and just use the sub for the .1 channel only. Is this possible to do with AV receivers -- to distinguish the .1 channel from the low frequency content in the rest of the mix? Am I missing something, or am I up against a limitation of the Panasonic?