At around the $750 mark per sub, here's how I would look at it:
If you want the most overall output and still have good sound quality, go with the XV15.
If you want the most linear response and still have a good overall output, go with the PB12-NSD.
If you want something with better sound quality than the XV15 and more output than the PB12-NSD, go with the VTF-3.
Rythmik subs have excellent sound quality, but you'd have to spend a little more in this case to match the subs mentioned above. An interesting note, I noticed inside a PB-1000 the other day that it appeared to have more than one set of wires going to the driver. That's not unusual with Rythmik subs because of the Servo, but not sure what SVS is up to.