To add an "it depends", not all subs will match well with all bookshelf speakers in all rooms. As n8wrl points out, there can be a gap between the two depending on cross-over, but placement, room size, etc. I have a largish room (23x16x9), a pair of M3 mains, VP100 center and TN-1220HO sub. I've never gone above -20dB on my receiver in HT, and usually go to -30dB, and I get decent response to 20 Hz. But I use a Rane parametric equalizer, without which there would certainly be gaps in the bass.
With well positioned floorstanding speakers, you'll get much smoother reponse down to the 50 Hz range with no equalization, than with a sub in a corner and a couple of M3s and no equalization.
So if I could be so bold, if you put an M3 on top of an EP350 in place of each M60 (same placement) I bet you'd get a better response than the M60 and a single sub in the corner.
Martin
