Subwoofer budget max $700: new vs used, brand?

Marty, Bob, Tgrisham, Ghstudio: Great feedback. This is the kind of feedback I needed. I would have take-away information from these exchanges, no matter what my purchase decision. I've used subwoofer's in my sound systems since the mid 80's, and altho I could have intuited the existence of the SMS-1, I was unaware that a reasonably priced device such as this existed. Pointing out that using one could be a priority, is why I talk to you guys.
I gave all your suggestions serious thought and decided to try to get the most woof for my buck in the speaker first, and then followup with the SMS-1 if I can't dial the thing in to my satisfaction. I'm a bit of a tweaker. My son and I spend a fair amount of time "equalizing the room" by ear, each time I change something in my system. If he and I can't get it right, there will be an SMS-1 in my future.
I decided to buy a new Velodyne Dls-5000r that I found at my price point. The fun will come in integrating it.
Honestly, the Legacy monitors sound pretty good alone. If the integration doesn't go well, it's not that long until Christmas :-) And it won't be in the budget to spend much more for a subwoofer any time soon, unless I win the lottery!

REL has a new-ish line of budget subs that you might want to check out.

You might also look at the Outlaw line. Good stuff at a reasonable price.

Long story short, I did not buy the Velodyne Dls-5000r.
At Larryski2's suggestion, I checked out the Outlaw line, and went with an LFM-1 EX.
I'm pleased with my integration efforts and very happy with the product. The Outlaw folks were great to work with, and the speaker was worth the 6 week wait to obtain at it's pricepoint with no charge for shipping (90lb speaker!.
Coupled with the addition of a Cambridge DACMagic to my system, my system finally has reached a more musical, fatique free character despite it's digital front end.
Larryski2, thanks for the suggestion!