I think I found an alternating sub that may work as a replacement for the Bose factory 10" sub in '03-'04 Y34 M45 cars. Of course you would have to re-amp this with a more powerful amplifier, but space-wise, I think these may work (that is if they can be properly bottom mounted with little to no fabrication on our part)
check 'em out:http://www.pioneerelectronics....ation
