I just upgraded the front stage in my 11 x5 with a Gladen One 201 Extreme speaker set and a mosconi 120.4 DSP amp. Its a real nice setup that works well as an upgrade to the base stereo in the 3,5 and x5. I've only run it a few days but its a huge improvment over the base system with allot more clarity, sharp highs, and strong bass.

In the x5 the thing thats a challenge is that the 4" mids have to be rather shallow to fit in the door, there are only a few models that fit (Bav sound, Focal's, Gladen's) without much modification.

The above kit replaces the 4" mid's, adds tweeters and crossovers that fit in the oem location and replaces the underseat 6" woofers with a 2 ohm 8" version. Everything is almost a direct fit but you need longer screws for both the 4" and 8" speakers and washers to raise the oem cover on the 8" woofers.

The install is pretty easy and you dont even have to take the iDrive out, you can just intercept the signal under the seat and run the Hi-level to the amp and then just run new lines for the 8" woofer (and use the existing lines to drive the door mids/tweeters).

One thing that is really good about the amp is that it has onbaord DSP which I would say is a must as the base system modifies the front signal (+6 ~50hz, etc, etc) and the DSP allows you to correct for this. Also the DSP level adjustment lets you tune the levels such that the gongs and PDC are as oem and not painfully loud as allot of people have had problems with the alpine upgrade. Also the DSP has a full crossover built in so you can isolate Hi/Lo freq to the 4"mid/tweeters and 8" woofers respectively.