It's not that I wish for SB-13+s in place of the DD18+, it is that if I were doing it over I would probably prefer two smaller subs over the one monster. I'm more than happy with the performance of the DD18+; it blends in perfectly with the Salon 2s and has transformed my system. But it is very large and very heavy. Inching it around to get the smoothest in-room response and using the crawl method was literally a PITA. Two SB-13+s would fit behind my main speakers, probably give me comparable bass for music, and the wisdom from the technically competent (like GranteedEV) around here is that it is easier to get smooth response with two subs than one. Not to mention something like half price.
Finally, when I ordered the DD18+ I was unwilling to take a chance on an internet-direct speaker I couldn't see or hear. Reading more feedback on this site about SVS, lots of which is pretty new, leads me to be more trusting that I could expect to be satisfied. I'm rather discerning about audio products, so I choose carefully.
Don't get me wrong, the D18+ is a great product, IMHO. I have no intention of swapping it out, I'm just saying I'd probably go down a different path if I had it to do over again.