For sheer extension, I would go with the ULS. For more output, I would go with the VTF15h. The VTF15h will get louder than the ULS, and in extension mode should be able to do 20 hz pretty well, this would probably be the best solution and most noticeable upgrade for you from a VTF2. If you are truly after serious depth, and can spend more then the shipped cost of either the ULS or VTF15h, I would seriously go for two VTF3s. That will get you very strong output at 16 hz. In terms of deep bass I think that will get you more sub 20 hz SPL per dollar than either the VTF15h or ULS-15.