You word things odd!
I'd get the VTF-3MK4 (sale price $659)over the PB-12NSD myself. Or if that is over budget which it probably is then look into the VTF-2MK4 (sale price $519) If the PB-12NSD cant be had for a cheaper price then also look at the FV12 for $500. Unfortunately no matter which of these subs you get you will eventually need duals.
Your space is a total of over 4,000 cubic ft. Just because your theater space is smaller doesn't mean the sub know that. The sub will be trying to pressurize the entire space & will get a bit lost because if that. So you need to approach this by filling the entire 26.5x17x10 space. Duals will be needed to have the correct headroom & dynamics.
A single VTF-3MK4 will sound decent IMO in there. Duals will sound great. Dual FV12s won't be bad & will run you just over $1000 in the end. A single sub in your price range can be had with the intent of adding a second down the road when you can afford it.
I'm not trying to say you HAVE to spend more but you WILL be
limited in your bass with a room that size & a budget of $600 just so you know.
Also, is that space open to any other areas? If it is the. You need to include that space with your measurements because the bass will be trying to fill that area as well.
For instance, my theater area is 13.5x19.5x8 yet is open up to a few large areas. The total space my VTF-15H/MBM-12 combo are trying to fill totals 8,600cuft. Not easy to do but I'm am very impressed with the bass my HSU subs put out. It did however take a lot of different placement trials & some EQing to get Flat. You will need to make sure you find & place the sub/s in an optimal location.