Something to think about with those massive subs is power usage. So those dual 15" Monoprice subs have 2kW amps in each. Do your power circuits have enough current to take advantage of that? If you wanted to bring two of those bad boys into the mix, I would bring in a dedicated 20-amp circuit for the subs at the very least. Something else to think about is logistics; it will not be easy handling those 215 lbs behemoths. The VTF-3 mk5s are very easy to handle by comparison. Also, if you ever wanted to sell the Monoprice subs, it would not be easy to do so.
If it were me, I would be looking at adding a near-field sub. It doesn't have to be another Hsu, if you wanted to try something different, but you would want something with comparable performance. Maybe a PB-3000, Monolith 15 or 12, Outlaw Ultra-X13 or X12, Paradigm Defiance X15, Emotiva Airmotiv RS13, etc.