GT,
You have ample space to fill. But the real question is how much you want to treat yourself with the "WOW!" experience of home theater audio...and therefore how much you're willing to spend on subs. Along with the center channel speaker, to my mind the subwoofer(s) is (are) the most important element in HT enjoyment. I have not heard the NHT sub, so I cannot comment on its performance characteristics. If you are satisfied with its output, but think you'll enjoy more...get another one. If you want to wish you'd warn diapers when movie watching
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, get a couple of SVS Ultras or Hsu VTF-HO Turbos. (Warning, the SVS's have rather large footprints.) And of course there are in-between options (such as my dual Hsu VTF-3 setup), and also much more expensive subs, as well.
Note that SVS and Hsu, if you give them a call, will make sensible (and trustworthy) recommendations regarding the model and number of subs best employed in your specific room. They're very helpful companies.
If you can talk the boss into the expenditure, buy what you want now. Too many of us will tell you sad stories of buying on the cheap...and then being unsatisfied and upgrading later...ultimately costing us more in the long run.
Just FYI, our room seems like it might be similar to yours...including galley kitchen and great room (with two largish openings to hallways), our space to fill is ~24'x28'xvaulted 8'-11.5'. So two VTF-3's fill 6,000+ cu.ft. quite nicely. I hope this helps.