It's a pretty big space to fill, but you have to start somewhere. I do think that a single PB1000 (I would definitely go with a ported sub) will make a difference. For one thing, it will give you another source for bass, which you can locate where it sounds best, as opposed to your front speakers, which have to be located in the front soundstage at more-or-less equal distances from you and from each other.
The SVS sub will also play lower than the towers, and at higher volumes. And, don't worry about using a ported sub for music. You are already using the equivalent of ported subs for music with your DefTech speakers. This will just be a better and more specialized sub.
One of the advantages of going with the PB1000 is SVS's free trial program which lets you test the sub, in your room, and with your other components. So, you can decide for yourself how much difference it makes. And later, as circumstances change, perhaps there will be upgrade opportunities (which SVS offers) to move to an even larger ported sub, or perhaps eventually to dual subs. But again, you have to start somewhere, and in my opinion, this would be a good place to start.
Edit: If I were you, I would ask people to respond to one or the other of the threads. It will be hard to go back and forth for dual conversations.