So the key to a successful UM18/4cf deployment is raw power to force it to dig down low even when it really doesn't want to because of the drastically undersized box and smooth it out with DSP. Similar is true with the VBSS but to a far lesser degree and the driver itself is incapable of dipping far into the inaudible range. Additionally, the Flex-12 is an excellent performer for it's size but because of it's size it's in a while different seismic category. Reviewing the charts though, it's not really far behind.
Thank you all for your help! You have given me lots of crucial info that really helps.
Glad you are looking into this some more =)
So a bit of a summary:
-The VBSS is a smaller ported box meant for a highly efficient but low excursion driver. The ports are sized appropriately for that. Ported boxes will have ~3dB additional output around tuning frequency for a given driver and power
-The PE kit you are looking at is a small sealed box. Sealed boxes have generally great frequency responses down to single digits... but are less efficient than a ported. They can be much smaller.
-On the flip side, a "full marty" is a giant ported enclosure. You really start to 'max out' the driver, per say, at the cost of a giant (4'x2'x2') enclosure. You'll still need DSP, but it is tuned for 17Hz and will put out a ton of output - from the same driver you are using in the PE kit.
So you gave basically one requirement: You want something small. The top two options above both give you something relatively small for an 18" driver. As they have the word 'small' involved, there will be trade offs - you just gotta decide what. They'll both outperform a 12" by nearly double in a given configuration, with being much less than double the size.
Now you have to decide what your real priorities are:
-The VBSS will be more efficient and have more output around tuning, but it will require DSP and won't hit single digits. It is probably still larger than the PE sealed kit, but the proportions may be better suited to your room.
-The PE sealed kit will be less efficient and will have lower output at certain frequencies, but it won't really require
DSP. Sealed boxes are nice, as they are relatively small and don't require that high pass filter to cut off single digits. However, in your situation, it doesn't look like you'll be getting 'audible' responses below 20Hz, so I wouldn't be too concerned about this unless you were looking to add on a lot later.
In my opinion, if you want the most output for a smaller box, both options are great uses of space per output. The VBSS uses a cheap pro audio woofer and does some pretty impressive things with it, so I personally would go that way as its a ~$100 driver vs ~$300 driver. If amp noise isn't an issue, I'd check out the behringer inuke pro audio amps if going that route - the DSP built into the applicable models is pretty good as far as subs go. You can do two channels that way with independent EQ if you ever want, too. Otherwise, you are looking at getting a standalone DSP if you go VBSS... something like a miniDSP.
Truthfully, two VBSSs OR two of those PE enclosures powered off of an inuke would be a really great setup for basically anybody not on this forum. You'd want the 6000 if you go sealed. If you can afford the floor space, I'd probably go with two VBSS, myself. Dirt cheap, solid output, and with two you'd get some benefit of having a more even room response. To get any appreciable output at lower frequencies than that, you'd have to spend a lot more money or cough up a lot more volume.