If the next sub you want to get is going to be equidistant from the listening positions, I would get a sub with roughly the same performance as the M&K that you have now. That means roughly the same output and low frequency extension. If you have the ability to EQ two subs, like in Audyssey MultiEQ 32XT, that gives you a lot more freedom. Although it's not usually recommended to run two different subs, it can be done if equalization and placement keep the subs' differing strengths and weaknesses in mind. Tell us what kind of bass do you want more of: deep bass, mid or upper bass, or just more bass output throughout. There are a few different factors that should determine what sub you should get, where it should be placed, and how it should be calibrated. In fact, depending on your budget, it may be worth taking out the M&K sub and just do an entirely new bass setup.