So quite a few people have installed a powered subwoofer so I'm not claiming to be the first, it's just there's no wiki article.
I've previously had a MY2006 (non-airbag dash) and that had a subwoofer fitted under the dash in the pax footwell. It seems the facelift dash (I have a MY2010 Exige) won't easily support something suspended from beneath the pax airbag so the logical decision was to place it behind the foot rest.
I had some help from pinkie and litton4 regarding their installs which helped me on my way but I don't think either of them put their subwoofers themselves so they provided a bit of observation and heresay from their installs.
I have experience of a previous installation in a Cayman of a Kenwood KSC-SW01, but choose a KSC-SW11 for this installation for 3 reasons. 1. the KSC-SW01 has a more limited remote control. This is not a problem if you have a multi-functional head unit that supports all the config for a subwoofer in the software (for the Cayman I had a Double DIN touch screen Kenwood DNX model) 2. the size difference is actually fractional. 3. The KSC-SW11 is a symmetrical design and so anticipating its location I thought it may look aesthetically more OEM.
The wiring for both Kenwoods is identical, and I planned to used phono leads to connect to the head unit. I have a fairly basic Alpine CDE-9872RM and whilst trying to resolve the archaic iPod interface it does support noted the phono for subwoofer connection.