Since the conversation has shifted to bases, just thought I'd mention something on the other side of the argument (against bases). If you do decide to remove the plates and have the subs firing towards listeners (as well as to change the height of a XV30), you may not want to use the original screws.
If you remove the base(s) you'll need to run out and pick up 6(or 12) shorted bolts to seal up the holes in the enclosure. If you try to re-use the current bolts they may dig into the insulation at spots because they'll be way to long. Use 1.5" long bolts.
I haven't had a whole lot of time to mess with my XV30s. System is pretty much untuned because I don't have the time to mess with it. Right now I agree with some of the other posters in terms of the midrange bass/chest thump being somewhat missing. Tom recommended turning the gain up and I did. For movies and stuff, it is pretty scary. I managed to crack some spots where drywall meets moulding. For music though, I got to really crank it (and I mean crank it) before I'm happy.
So I'm positing a few scenarios -
1) Subs are sitting in null spot, or I am. 2 subs though... I dunno
2) Like Tom mentioned, these like a lot of gain. The inbalance between movies/music is large though and I don't like going back and forth tweaking the gain between media
3) Speakers are not level matched by far so EQ'ing + running room correction should help
4) I may be getting too much low bass, that it's overpowering the other frequencies, making me overcompensate by cranking it yet more. Again like 3) got to take the time to tune.
On a side note, I cannot imagine a pair of XV30s on a subfloor. They're quite capable of scaring me on a solid concrete slab/basement with shaking. Just wishing for more from the impact of drums/percussion/eurotrash dance music (ok I have no taste :>)