I installed a 8 inch subwoofer in a sealed box I fabricated under the
driver's seat. There is about a 5 mm. clearance between the bottom of the
seat and the speaker grill guard. I described the box in some detail
previously (in the archives). It is fabricated from 1.5 inch thick MHD
particle board. Measured low frequency cutoff is around 37 Hz. I drive it
with a 250 watt rms amp located under the passenger seat. The amp is cooled
with a whisper-quiet fan. The sub box takes every cc. on the compartment
below the driver's seat. The amp is mounted forward of the storage area, so
no impact  on storage volume.

'82 Diesel Westphalia converted to 1.8 L 8V GTI Digifant.

Hope this helps someone,

Frank Grunthaner