You should just go with an after market system and get twice the sound for 2k !!! Rockford T1000-4 amp $750
HERTZ component system $600 with crossovers
Selenium Tweeters $80 for 2
CAPACITOR 2 farad $120
Installed by professional $250
Now you have money left over to go take your girl friend out
YOU dont want a custom BOX built for subs because that is like having a passenger in your car at all times weighing your car down and will kill your GAS and reduce your speed because of the weight
They use MDF wood which is heavy as shit and goto home depot and try to lift one up by your self
! im just saying its heavy so you dont want that type of weight in your M3 !! Maybe a 325 or 330 but not a M3 !