I've really found this thread (and others on the site) helpful and figured I'd share a bit of what I learned abotu all of this.
I've got the great pleasure of spending $1500 to replace this amp due to the drainage issue as well. Turns out plant debris plugged the drain in the convertible top and afer a car wash last Friday it went out (fortunately, due to the dryness in AZ it dried out and returned to working order). Then Monday we got rain and it went down for the count.
Had I known about the issue (and the resolution) I could have cleaned the drains at the first sign of trouble and, well, my wallet would be $1500 heavier. Better yet, I'd just keep the damrned things clean.
So, folks, I guess there's something we put on the annual check list - clean the convertible top drains. I'll be whining at BMWNA as given the problems with the drains these should have been checked as part of the service procedures.... but I expect little...