With subs the more high volume output you want the less deep they will play and vice versa for max extension.
The EP350 specs quote it's actual response in an anehoic chamber where it plays down to 28hz but that 28 is 3db lower than the rest of the frequency range. In a room you will get lower than that as the room helps boost the lower range so you could expect to hear/feel around the 18hz range, but the 18hz is 9db lower than the other frequencies.
The Hsu sub has different settings that allow for maximum extension - 18hz or max. output 25 hz. I suspect the 18hz is not an in room guesstimate as it is with the EP350. In room response with the VTF-3 could be closer to 15 HZ or lower. It's hard to say as all rooms are different. I took this from the owner's manual: " "Now you decide how deep to play.” The large flared ports can be re-tuned with the flip of a switch and the
removal of a port plug or adding of a turbo. In Maximum Output mode, the subwoofer transforms into a different
device capable of playing louder material with less distortion at the expense of some deeper bass"
