I'd give another nod to the babyboomer. I still use one with my Soliloquys, and I used it previously with the MMGs before I sold them. Another one of the same design type you might want to look into is the Sunfire True Sub (and True Sub Jr.).
BTW, if you do a lot of searching around, you should be able to find one vendor that sells the babyboomer for about $500, otherwise the street price seems to be in the $700-800 range. Depending on your room size, you could also look at the smaller Pinnacle subsonic.