But you really should try moving the speakers around the room to see if you find that bass you're wanting, before investing any more money.
Why? That's where the Polks were and they worked fine!
Honestly, it could be also that the Polks might not have a flat frequency response somewhere in the 50 to 80Hz range and might have accentuated the bass effect and the M80s are flat and you're not used to it. I get more of the "punch" or "feel" when it's handled by the M80s (crossed at 40Hz) than when it's handled by the EP600 (crossed at 80Hz). If you don't like the way it sounds, first play with placement as Chris suggested, then if you have some kind of EQ, try increasing the level on the 40/80Hz range, then give a sub a try.
