It's not so much about making the house rattle, its more about making sure that the listening area has no dead zones or issues with standing waves - there's a lot of misconceptions about bass nowadays. Others could get more scientific about this. This statement is not aimed at you personally though.
For you, my good sir, I would think about the size of your room in square feet as well as its volume as well. DefTech makes excellent subs - if anything, upgrade from the ProSub to the Ultracube instead of getting a pair.
Edit: Saw your HT photos. You're fine!
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