I have never heard the box in a X5, but I did install one of these in my fiance's Grand Cherokee. The bass that it puts out is amazing, nice and tight no rattle. The amp used to push her's is a Rockford Fosgate R500X1D 500W RMS Monoblock Single Channel Class D Prime Series Amplifier.

Rockford Fosgate R500X1D 500W RMS Monoblock Single Channel Class D Prime Series Amplifier

The amp fit the parameters of the stealth box perfectly, 500 watts RMS specs, but from the factory it was tested up to 623 watts RMS. If you want great sound in a tight package I recommend this setup for your X5 as it's a compact amp. Either way the stealthbox was a great set up no matter what you use to drive it.