I'm a little surprised at anyone experiencing clipping or bottoming out with their X5 or X3. I don't typically listen to crazy deep bass, but on occasion I'll play something that digs pretty deep, for example Macklemore's Downtown, Tyler the Creator's Earfquake, Mussorgsky's Pictures at An Exhibition, or Stravinsky's Three Dances from Petrouchka, and I've had no issues with my own X5s bottoming out, and that's with the active woofers only about half-way up.
I test drove the X5, X4, and M3 when I was deciding which to get, and (though I'm now more remembering what I thought at the time than what I actually heard, aural memory being what it is (then again, if what Nietzsche says about memory is true - "If memory says I did but conscience says I couldn't have, memory loses!" - who knows if I'm not remembering things in a way to feel good about my choice)), Nietzsche notwithstanding, I thought the M3's bass the biggest, but I also thought it a bit bloated and overdone compared to both the Xs, at least in the relatively small Spatial listening room. I didn't find the difference between the X4's and X5's bass significant enough to choose one or the other because of it, so I went with the X5 for its greater sensitivity and consequently wider selection of amplification possibilities. As I say, I've yet to exceed or even reach the limits of how low the X5's bass can go. No doubt someone out there has a recording in mind that will make the X5s tap out, but, if I have to go that far out of my way to find something that will make them hit rock bottom, it's not going to trouble my hifi slumber.
On a related subject, I thought clipping was a product of either mismatched impedance of amp and speaker or of an amp simply lacking the power necessary to drive the speaker at high volume. It's the amp that does the actual clipping, no? Or have I got that confused? Also, is bottoming a speaker out - getting distortion at the low end - a form of clipping? I ask because I flat out don't know.