Subwoofer Suggestion

Testing it wouldn't be possible. So since dual sb or pb1000 is underpowered would it be better if I go for a 15inch PSA S1500?

Can you return them?

It would be underpowered for me, maybe not for you. Forced to choose, I'd take two small subs over one big one for that sized room. 

I have quite a large room (28'x15x10-14', my ceiling slopes a little). I've rearranged this room a bunch of times and have had to find different places for my sub (REL T9). I only use it for listening to music (no TV). I've used the step by step set-up instructions from REL every time I've rearranged this room. I've been able to get amazing performance out of this sub every time. I think this speaks to the quality and thoughtfulness that they put into their products. There's one additional step I take setting up my REL. If you go down that road, DM me and I'll tell you about it. 

I recently added a stereo pair of REL T/9i subwoofers to my Magnepan 1.7i pair. They have revolutionized my system. I’m a believer and vote for REL. Use the high level input. Good luck. 

For stereo I use dual Rythmick F12 sealed  subs. Loads of adjustments to make them disappear into the speakers. Plenty of power at 370 watts each with very tight musical bass. They have ported subs better fitted for HT.