PB12-NSD VS. Hsu Vrf-3 mk4 for home theater

If you have 8 foot ceilings, that's 4320 cubic feet you're trying to fill, not including the adjacent open space. Given your budget, you might look at something a little more robust, like some of the 15" offerings from Hsu and Power Sound Audio. The differences between the subs you specifically mention in your post are well documented. My space is less than 3000 cubic feet and I'm considering the Power Sound XV15.

Either of those subs will destroy your Paradigm sub, there is no question about that. As for the differences between them, the VTF3 is larger, likely louder and will play deeper. The PB12 has extremely low distortion and is well behaved due to the digital amplifier. It won't bottom out no matter how hard you push it. The VTF3 has different tuning configurations so you can shape its sound, whether you want deeper bass, or tighter bass, or punchier bass at mid 20 to 40 hz region. I think the PB12 has a better warranty, but both subs are very reliable so you are less likely to take advantage of a warranty with these than most other subs.

On your budget, you could get a PB12 NSD or VTF3 now, and if it isn't enough for you, get another. Or you could get one big sub like a Rythmik FV15, Hsu VTF15h, SVS PC12 Plus, or PSA XV30. Alternatively, you could get two Hsu VTF2s or Outlaw Audio LFM-1 Pluses now and stay under-budget while enjoying the benefits of a multi-sub system (better room response and no sub localization). The VTF2 and LFM Plus are pretty powerful subs themselves, probably only a couple DB under the VTF3's max output and a few hertz short of its extension. A system like that might be more appropriate for a room with a broad seating area.

No, forget sealed subs. They don't start punching hard until you get into the more expensive ones like the Seaton Submersive ($2.5k), or high powered multiples like Hsu ULS 15 duals or SVS SB13Ultras. If you liked getting kicked in the chest, you will either need a very powerful sub, or a near-field sub which is a sub that is placed very close to the listening position. If you can't accommodate near-field placement, I would go for the single most powerful sub you can afford, given your criteria. If you can stretch your budget a bit, check out either the Rythmik FV15HP or Power Sound XV30. On the other hand, if you can accommodate a near-field sub, something to think about is getting a VTF3 and setting it in the corner in deep bass mode, and getting a Hsu MBM-12, and placing it behind your seat. Those two will come out to just over 1300 shipped and will unquestionably give you that punch in the chest feeling, it's what the MBM was designed to do.

Audiogon will have these kinds of subs from time to time. Another place to check out is the audio classifieds here at Avsforum. Other audio message boards have classifieds too, like Audioholics.com and hometheatershack. If you are in a populated area, you can get lucky on craigslist at times, and ebay also usually has one or two of these kinds of subs going, but unfortunately they are in such demand that the auction price is frequently too close to the new price to make it worthwhile there.