I couldn't find I shop I would trust or that wanted to deal with a small scale
(according to them) upgrade.
I found most of the shops had little E9x knowledge and a very limited amount of anything that would fit.
After doing a ton or research and lurking on the audio subboard for quite some time I probably know every way you can put one in wrong and how to avoid it. On the monetary side I probably saved about 1k in installation costs. And I can now support or modify my system without assistance.
BSW won't help much with the base audio as there is no amp and the
power output of the head unit is only about 12 watts if that.
There system is optimized for L7.
Also the base system head unit has a equalization built in to make up for the lack of tweeters that causes you to need a processor to straighten it out before applying an amp real tweeters and better speakers.
Go here for audio answers.
http://www.e90post.com/forums/forumdisplay.php?f=27
You need to go read the links in the first few entry's in this thread where I showed the musicar system examples. Like all the links in #3