I'd go with the w6 since I have personal experience with them and they simply are hard to beat for SQ. (not too sure how it'll like 1000wrms+, just set ur gains correctly)Also look into the Dayton Reference series. I run these now and SQ-wise they rival my w6s but not as loud.
Both are very transparent even up to a 90hz xover point and still have had pretty flat FR in my setups. So, thats a little more forgiving in case your front stage cant play authoritatively down to 50hz or whatever. (which is something I look into when looking for an SQ sub, not just one that can get low and sound decent)
I'm pretty sure an Fi Q recommendation is coming but it wont touch the w6 or Dayton for SQ.
Image Dynamics also supposedly makes good SQ subs, but I have no personal experience outside of hearing one for a quick second.