ok, you see some people say that sealed subs are tighter than ported subs. but this is too generalizing, because it depends on the design and quality of the components. for example, when Wayne reviewed the SB12 Plus, he said he couldn't hear a difference between it and the PB12NSD (excluding extension of course).
some people can hear a difference, some people don't. I for example heard a difference between my ported Axiom and sealed DD. but alas, I preferred the booming sound of the Axiom especially for R&B songs. (even for the Eagles Hotel California)
that said, I'm sure your SVS plus is better than the NSD model and thus better than the SVS SB12. so, there shouldn't be anything in the SVS lineup to match the Velo DD in terms of tightness and ultra low THD. (the built in EQ is a very big plus btw, if you fix your room frequency response, you could find the sound you are looking for)
options:
1) use the REW/BFD on your SVS to correct the peaks and dips
2) get a Velo SMS-1 to do the same as no.1
3) get the DD-12 (or bigger)
4) get the SVS SB12 for a home demo, return it if you don't feel it's an improvement over your existing SVS.