I have an '06 3i with the factory 6 speaker, 6 CD stereo. It is probably the best factory sound system of any car I've ever owned, but there is still plenty of room for improvement.
Can anyone recommend a decent sounding set of speakers to replace the originals with, or any other mods that can be made to the factory system?

 

i have infinity Kappa's, and they sound pretty sweet

 

I installed a JVC 12CD mp3 changer and a 10" sub in the trunk and I'm happy with it.
mp3 changer -- http://www.jvc.ca/product-images/CH-X1500RF.jpg
sub -- http://akamaipix.crutchfield.com/pro...7ATB8AT-f.jpeg
Just FYI I paid just under $300 for both (brand new). Both came with all wires etc.. ready to install. The way I installed them you can't see anything in the car except the sub if you are close to the trunk when it's open. Maybe I should post some pics. AnywayZ... good luck!

 

haha... a bazooka tube....nice
i have 2 12" JL12W3's in my trunk, but my avatar gave it away eh?
anyways, they make crazy bass, and i just turned them around, so the subs face the windsheild now. it def. made the bass more clear

 

Audiobahn not Bazooka
A few reasons I didn't put 2x12" (for example) is I still wanna have a trunk space as well as keep my ability to hear. Believe me, you don't wanna go deaf. Also, 2x12" are way heavier than what I currently got.
Nothing personal tho
Enjoy your sound as I am mine