I saw this the other day I thought it was interesting.
http://www.rockfordfosgate.com/News/....aspx?id=13744
Here are a few other options.
http://www.jehnert.de/2011/09/28/bmw...unting-places/
http://www.mbquart.com/mbquart_bmw_speakers.html
I have Morel Hybrid Ovation II's and Jehnert XE200 underseats. They sound great with any type of music, and I don't think they are harsh at all. The Morels and Jehnerts are a really good combo if your not on a tight budget. I recommend an amp that does at least 100 watts rms at 4 ohms if your going for sq.