SVS and Hsu are both quality brands.
Questions:
How big is the room? Is the room sealed or open to other areas? How loud do you want them to be able to play? How low do you want them to be able to play? How big of a sub is acceptable?
How do you use your system? All music? and if so what kind? Pipe organ and or electronica are much more demanding of a sub than Classic Rock or Country...
If your usage is action movies at high volume you'll probably want a larger more powerfull sub than if you mostly watch drama movies.
Depending on the answers to the questions above...Maybe???
1) Room size is just over 2500 cu ft, so not huge.
2) The room has one opening to it, and it is a stairwell.
3) I'm not quite sure how to answer this. It's movie dependent. However, I have set them up with the recommendations from HSU in the one port open mode. Q setting at the lowest and volume at 9 o'clock. Which from 1-9 with 1 being lowest they are set at 3 for volume.
4) so with some movies that are heavy bass, I've noticed the sub dropping out and chuffing or whatever you guys call it. so I figured it would be a good time to upgrade.
5) maybe this answer is biased at what subs I had in mind, but the size of the VTF 15 MK 2 is probably the largest I could get away with before all hell breaks loose with the wife.
6) I watch TV and movies mostly. I'm not a gamer. I do amplify my guitar through it occasionally.
Also I use a lower end Denon receiver AVR-S920W