Adding a second pb1000 down the road would be a great idea. IF you felt the need
1500 square feet!!! I hope you meant cubic feet
One pb1000 will be impressive in that small of a space. Two would be awesome. That's the great thing about having a small dedicated space.
sorry if I took your post the wrong way, but if you are experienced in setting up three subs then you certainly know what to do for one.
You can get decent response measurements from one sub. As good as with two subs? Well, of course not but you can get it to a level where you can be happy with. Well maybe YOU can't, but every situation is different.
As long as your open to having the sub where it performs best, not just where wifey thinks it looks best. Good response is possible
WAF holding back your sub locations? That's a whole other story and another plus of a dedicated room.
I know your pain. I have fought the multiple purpose room battle of performance vs. WAF myself. I'm going to have to live with two pb1000's in a space over 10k cft. For crying out loud (cry me a river, right?). But I'm accustom to my quad 18 setup in our current house.