Perhaps you could get another sub & hook it up in truly 2 Channel (Stereo) mode.

This is what I do with my M2s Sats plus a pair of EP400s Subs hooked up to my Tube Amp from the amp's 'speaker out' connections to the 'High Level In' on each sub - not through an LFE channel where both channels are most often combined.

This way they are truly 2 CH Stereo with the nice ability to dial in the bass response the way that makes you happy....

TAM