tobz67,
Are you sure you've read that the VTF-2 can go lower than the SVS PB1-ISD? I thought that the PB1-ISD is more comparable to the VTF-3, rather than the VTF-2. Indeed, with respect to the specs and physical size, the PB1-ISD is quite similar to the VTF-3 in the Max Output mode (two ports open).
Personally, if I had to choose between the VTF-2 and PB1-ISD, I would probably go with the SVS (assuming that you don't mind the bigger size). The VTF-2 is surely an excellent sub, but I don't think it defies the basic laws of physics -- the fundamental designs being similar, the larger the box, the deeper and louder it plays.
