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, Loaded except for that stupid cruise control and moonroof. Mods: AA pulley and air filter. Fabspeed 200 cell cats, Xpipe w/resonators and no secondary cats, Powerchip tune, Eisenmann Sport Mufflers, Xenon interior lights, Vorsteiner Rear Diffuser II, SWS8 subs, JL Audio XD400 bridged 2X200W