SVS, HSU, ect are all good choices. But, you mentioned "boomy" was a problem and that you might want some resale. So I'm going to recommend a sub that I think is very nice, even if it is not a big as some that you were looking at, the Velodyne Minivee
http://www.crutchfield.com/S-stdHncC...sp?i=232MINIVB

At $800 retail, maybe a little more then you want to spend, but I think a speaker as refined as the Studio 40 deserves a little more. Actually, you can get it for significantly less on Ebay. Come to think of it, with the 40s, I would pony up the extra cash and I would go for the SPL-1200R with automatic room correction. http://cgi.ebay.com/VELODYNE-SPL-120...em180028452738

That will kick the hell out of your upgradeitus.