Depending on how the sub is tuned with the mains and the quality of it, a sub could improve things. It may improve them even more if you bypass the very low frequencies so they no longer hit your mains. A good sub has that capability built in (you need more cables though). Or, you could buy an active cross-over, which should produce better results. The objective is to remove the very low frequencies from the mains so they can, if you will, focus on what they are really designed to do -- mids and highs. For example, a very low frequency input to your mains moves the cone/ribbon/whatever by quite a bit. In the meantime, the mids and highs are also moving it. A dopplar effect occurs as, for example, the lows are moving it back while the highs are moving it forward at a lower rate.
At the end, try it ... you might like it. Let your ears decide. Try it first with just a sub (see if you can try one at home before buying) and get an active cross-over later if it makes sense (check to make sure it matches up with the Maggies though -- I believe they have a different kind of capacitance load).
