My '03 X5 has the standard stereo. I opted for the Kenwood wOOx, a very small sub/amp product that takes up very little space and includes a remote that gives some control over frequency and volume. However, small drivers (one is passive) and 50 watts don't give you much. My backseat passengers like the "fill" it provides. To be honest, I don't notice it much. Oh well, I think I spent no more than $150 bucks on it and did the install myself. If I had to do it again, I would probably go with a custom box in the factory sub location.

Like you, I was only looking for an improvement in bass response, not anything obnoxious (BTW, I'm black. Where I'm at, it's mostly "other" folks who have the 'boomin' systems' in their accords, scions, and other imports).