Well Jake, adding two more sealed 15's in addition to what you have now should give you 3db. It would also give you an even smoother response in your room by spreading out your sources of output even better than with just 2 enclosures. I guess the question is whether or not 3db is "good enough" for you or if you'd desire more down the line. Going with two 18's, still sealed, is only a marginal cost over two 15's generally speaking and would probably give you almost 5db over your current setup which is pretty nice. If you're looking for that 6-10db gain, you're going to have to go ported but again I'm not sure how that would blend with your sealed enclosures; I've never mixed them so you'd have to ask some of the more qualified/experienced guys on here their opinions on it.