IIRC, it is about 2.5 inch clearance
there is also a grounding stud that would need ot be cut off and teh ground point reattached somewhere else
The space is the same as under drivers (left) seat where the ECU is
The best solution would be a powered sub like
http://www.ebay.com/itm/KENWOOD-KSC-SW11-COMPACT-POWERED-ENCLOSED-SUBWOOFER-SPEAKER-AMPLIFIER-BASS-BOX-/121080886478
but less height
Its about .25 inch too high. I was looking at raising the seat tracks with some spacers, but the way they are mounted with a bolt at an angle in the front and flat at the back would make it difficult to just add a .25 in spacer as it would shift the track forward
Maybe a new project for you ...LOL
Like many Boxsters, I don't have Bose or door subs and also no amp, so a powered sub would be ideal