DIY is a very good way of getting more bang for the buck than a commercial offering.
However, I’d still choose the SVS because
they sweated the details in tuning it. While we haven’t seen the PB1000 measured by an independent source, SVS is very careful to tune their custom DSP amplifier so that it never makes bad noises when pushed. You’ll never hear the sub clipping nor bad port noises because once the sub reaches compression, it behaves cleanly
Look at the larger PB12 http://www.audioholics.com/reviews/speakers/subwoofers/pb12-nsd/pb12-nsd-measurements
specially look at the compression frequency response chart and also its distortion charts. This is as good as it gets. It’s clean (low THD, and the frequency response is flat as a ruler). Thiry amp also has a custom EQ curve to give it the right boost in the lower frequencies that a standard amp won't have. Look at the max long term output chart to see what I mean about clean limiting.
Sure you can build a sub yourself that puts out more dB’s, but will it sound as good? There are probably ways to equal or beat the PB1000, but what are you after? Super high dB's for low bucks or just clean room filling sound?
Buying a SVS is about design, not just slapping a driver into a box. SVS started as couple of guys making DIY subs as well, but their years of experience help shaped their good designs
If you are serious about beating the SVS and making a good sounding sub, go into the DIY part of this forum. I would start there rather than what car audio guys are suggesting.,