First you've got to understand there's a ton of trolls asking this vs. that questions about their favorite woofer.
Both the Velo and SVS are great subs, but for totally different applications. If you want a great musical sub, small footprint, medium to small room the Velo has great features and will fit the bill. If you have a large room, want extreme extention and SPL, don't care about how big the sub is and want "oh my lord" response, then the /2 is the ticket. It's still a very musical sub, but the HT response overshadows most reviewers opinions.
Frankly, I'm amazed by both products, though I haven't heard the Velo SPL series in my home, I have had the DPS and DD series here and they are great. I also own a 20-39+. As I said, it ain't apples to apples...it's what is important to you.
Good luck (SB12Plus coming in Sept - which is apples to apples)