Most people with a L7 just add a sub and sub amp(10" or larger trunk sub)
for big bass.
subwoofer threads
http://www.e90post.com/forums/showpo...2&postcount=66
http://www.e90post.com/forums/showth...t=Ultimate+Sub
http://www.e90post.com/forums/showth...ight=subwoofer
when you say best do you mean best or most cost effective.