If you get much room gain adding to your subwoofer's low end, the exaggerated deep-bass is going to muddy up the mid/upper-bass. If that sounds like part of your reasoning to cut the subs out from your music listening, read on.
If you want to experiment, you might try connecting the M8 instead of the SVSs, run Audyssey, and see how it sounds as a 2.1 system.
The theoretical advantages (of the M8 to no sub) are that you are getting a good solid 30-40Hz range that your F5 doesn't provide, you are relieving your mains of trying to produce those lowest frequencies (which is pushing it's limits), and, of course, you are relieving your AVR from needing to power those lowest frequencies (set XO at 60Hz).
The theoretical advantage (of the M8 to the SVS subs) is that your M8 rolls off at a higher frequency which would counter-act the effects of room gain to some degree. It doesn't need to be a perfect compensation, just close enough to get the level of in-room bass flat enough that Audyssey can flatten it the rest of the way. The M8 will never give you a solid 20Hz, but that is rarely a concern with music.
