Sealing your setup from the trunk may or may not gain noticable results. It will guarantee that you spend a lot more time and money installing it if you want it to look anywhere close to decent. If your running 1 12 or 2 8s the box will be plenty small either way, and will get plenty loud without sealing. If your trying to squeeze 2 15s or something where the box takes up the entire trunk sealing it off may be a better idea. The problem with sealing your box into cabin, is if there is one leak anywhere, the amount of output you lose will be much greater not sealing it to begin with in many cases. There are plenty of loud trunk cars hitting respectable numbers without sealing off the trunk, so unless your a hardcore competitor chasing tenths, I would recommend a nicely designed ported box. Box, sub, and port position will vary from vehicle to vehicle, but I would either try sub facing back, port back (toward the rear of your trunk), or sub forward/port foward with the box as far back in the trunk as possible.