I've been thinking about Jon's idea of under-seat mounting for a while now. Or a variation of it at least based on the e46 where they still have underseat air ducting so directly underseat not an option.
When I did my m3 install I found the e46 has two deepish cavities that sit between the sill / seat / and heal space. The area has a hard foam insert that just comes out. Looked like enough air space for a 6.5 to 8 inch woofer.Was thinking of something like a Gladen woofer (shallow) and a fibreglass enclosure using a e60 woofer cover or similar. Would bring more mid bass to the front.
Having seen the de-evolution of the wife's car audio system (started with Sony hu with good preamp specs, 6.5 comps , ribbon tweeters, 10 inch sealed sub , 60w and 300w amp to just HU powering and front comps) you quickly release that space becomes the driver and anything that takes boot space has to come out :-(
Anyways Olaf, I see this been the evolution of my system so thought id share. I'm not going to cut the M3 doors, but for your install could solve the bass question without giving up the rear cargo space.
Anyway .... got to stop these day dreams and finish the damm Mini ........
On the sub front its personal taste and so many factors in cars. Sometimes you want that perfect bumph out of a kickdrum ...other times you just want to pulverise the living s..t out of your senses and re-live being 10 meters way from the W bins at that Faithless concert.