Tried the sound deadening, inside & out. It actually made it sound worse, but that was on an OE Bose sub, which uses the same enclosure but at least has a 100w amp & two 4" drivers. Had it wired up in the workshop and the whole enclosure would vibrate quite quickly along the bench at full tilt.
The amp really needs to be 4ch as the non bose sub amp also drives the rear speakers. You can pick up the pre-outs and remote on from the OE sub loom, let me know if you need the pin outs and/or instructions on how to remove the unit.