Good to hear some others have tried the Boston's, I'm also considering a set of Rockford Fosgate's. Knowing that there are some reliable 2nd options out there, I'd just as soon try out some other speakers to see if we can increase the number of "known-good" configurations. If they don't work, then at least we know, and I know I can fall back on others to move forward.
I had read some of the older threads about stereo upgrades, and it baffles me the mentality of some folks who think that something that increases the enjoyment of your vehicle as much as improving the stereo does is somehow some kind of blasphemy in a BMW. It's just a car, right? I'm with you though - give me something that's worth listening to! I've never kept the stereo stock in any car I'v ever owned.