only place you can hide subs (as stated above) is in the spare tire well, been there, done that, too old to have to call somebody when a flat tire arises. there is no room under the rear seat, and there is next to no room under the front seats, i have had a couple of customers want amps mounted under there and there not be enough room. as long as your not looking for windshield shaking bass (which you wouldn't be able to hide anyway) you can get a lot of good bass out of a little space with quality components, such as a pair of 8"s or a single 10" or 12". a single 10 or 12 wont take up too much space and a pair of 8's and a small enclosure can be mounted under the rear deck lid.
the Subaru's have an option(dealer and factory) for a under the seat sub or two (if you really want to call a 6.5 woofer a sub in a flat box) not to mention the seats are much thinner and lighter in an STI and that they dont have motors and junk inside them