Why do a stealth box? I may add a trunk sub using JL too. My guy will build a nice enclosure that goes in the back left, over the OEM amp. That way, I get to fold my seats down and I don't lose any real trunk volume. I do use the trunk.
Although nice, the Stealth Box is expensive and compromises the trunk, greatly.
2009 Space Gray Carbon Leather E90 with DCT, Tech package, and EPS. Mods: Fabspeed 200 cell cats, Xpipe w/resonators and no secondary cats, BPM tune, Euro DCT software, Eisenmann Sport Mufflers, Vorsteiner Rear Diffuser II, SWS8 subs, JL Audio XD400 bridged 2X200W, built in radar