Hi, Junior. Do you have any subs now? If so, how do they sound in those cabinets?
My brother has a sub placed inside of a built-in very similar to yours, and he seems happy with it. I agree with John that it's not optimal - but it might still be enjoyable.
I wish that I knew how to answer your question, but it's a tough call. If it were me, I'd probably go with the dual SB12s because it gives you a better chance to smooth out the response over the room, and room gain should help you get more output in the lower frequency range. I agree, though - ask SVS what they think. They were always good about helping me out when I was shopping, and never tried to talk me into anything more expensive.