Unfortunately, we don't quite know how the methodology in how they got that response curve, so I cannot really comment on that. However, we do have freq response measurements on our site for all three of the subs listed (measured ground plane from 1 m away from the front baffle) and you will be able to see how one sub compares to the other in terms of freq response and extension. All three subs would be pretty similar in extension. I would say that the VTF-3 Mk5 HP would be a good upgrade if you're looking to get more clean output from 25 Hz on up. Below 20 Hz, both will fairly similar, with maybe a bit of a nod to the VTF-15H due to the larger cross-sectional port area (less port noise). The VTF-15H Mk2 will be the one to go with if you're looking for more improvement across the board. Still, the VTF-15H Mk1 is a great sub.