I have had good success using a B&W ASW-700 sub crossed over at 50 Hz with my S805's running full range. The ASW-700 plays quite deep (-3 dB at 22 Hz) yet it's not overpowering and doesn't muddy up the sound like other subs I've tried. Also, it doesn't take up much room and looks very nice, as it's available in a variety of real wood finishes - not just black. My room size is slightly smaller than your's, so you might also consider the ASW-750 or the new ASW-825.
