I’ve installed subs in several of my past BMWs that didn’t have good low end and never had a bad issue with vibration. I even built a custom enclosure that fit in my spare tire we’ll just like you’re talking about for an old three series. Couple rattles here and there, but I was after quality sound and not boom noise and with a little bit of finesse got rid of any issues. I haven’t messed with anything on the X5 though. It’s not theater quality or anything but it’s fine compared to what I used to have to modify.