The MusicarNW systems are made out of audiophile name brand components.
They are not add on's to existing systems they are complete systems with matched amps , speakers and other components .(in which all speakers can be replaced) So far for the e90 BSW only makes door speakers which are improvements on the stock ones.
Yes they have a subwoofer add on but nothing for the under seats that I am aware of.
http://www.e90post.com/forums/showthread.php?t=641323
There are many examples of there systems in the above link along with the knowledge you need to make and intelligent decision on what to do.