Talk to Tom Vodhanel from PSA about the XV30 before you start to take peoples "hypotheticals" on what it can and can't do at 12.5hz compared to other subs. He'll give you an honest answer. Tom was the "V'" in SV Sound before he left so he knows his stuff. Just go over to the Power Sound Audio thread and shoot him a PM.
This is what Tom said on the PSA thread a few days ago:
The XV30 is essentially a larger, more powerful XV15. The sound quality and extension will be identical or at the least *very* similar. The only real performance difference is the maximum output capabilities. The XV30 will have about 4dB more headroom. Another way of looking at that is a single XV30 in a front corner will perform nearly identical to dual XV15s with one placed in each front corner. So basically, if you want to know how a XV30 "sounds"....just read a XV15 review and factor in the additional output capabilities.
Tom V.
Power Sound Audio
This data and Tom's response is why I said the XV30 (if like the XV15) would likely not be able to produce a 12.5hz signal (or one with significant output). It's less "hypothetical" and more from the horses mouth, so to speak.
Powersound makes great subs, and both the XV15 and XV30 are great examples of that. However, like all subs, there are design compromises. Extending to the 12.5hz realm and below was one of them. Note that this is by no means sub par performance...most ported subs can't dig as low and loud as the PSA's. Just when compared to the FV15HP, it does not extend as low. But again, that's where I believe the FV15HP separates itself from the rest of the commercial ported subs.
HTTYD for one 
I'm flat to at least 5hz (can't measure below that). When you've got reference level playback at very low frequencies, you'll never settle for anything less.
I have another 9 days til I can order it.
