I had a similiar idea but didn't want to lose the storage so I fitted a Blitz 10" powered sub woofer to the side of the boot behind the near side rear seat it's 372 x 275 x 76 deep so it does not protrude into the boot space by much. At the same time I installed an aftermarket 7" sat nav/dvd/tv/radio I bought from a Chinese dealer on EBay. It has a dedicated sub woofer output. The Sub was $150 free delivery, it works well and takes up less space than others, plus they supply all the wiring, you just need to install it.(a fairly interesting task)