I have just bought a 18d SE and have done loads of audio upgrades on the current gen BMWs, did our 535d, the 335i, the 320d, 120d and 123d.
I did an install guide for the 3 series which will show you what comes apart, all these cars are the same really.
http://www.e90post.com/forums/showthread.php?t=136594
I think I will add a Vibe Litebox Stereo 4 to mine, these are great little Class GH amps, smoother than the Class D amps and they don't mess with FM reception like Class D does.
If it is still lacking bass I will put some Earthquake subs in the stock locations, theses are awesome subs that go down to a proper 28hz.
Oh, will also add the Logic7 tweeters, but will keep the stock mids, once driven properly these are fine, actually prefer them to the L7 mids, which are a bit bright and what you get in the alpine upgrade.