@ryder, Thanks for the kind comment.
I looked long and hard at SVS SB-3000 subs, I read many reviews and peoples comments about them and determined that SVS was a better fit for H.T. This is strictly my view.
The most common complaints about the RELs are no numbers on the X-over and volume settings.....completely agree.....have to count the clicks. The other being no variable phase, I thought this would be an issue, since both JL and Rythmic offer it but so far no complaints.
For me, the REL's were very easy to set-up and to get Intergrated with my mains, took about three days.
Lastly, all of my past subs have been 12in, I wasn't sure I wanted the S/510s, I contacted REL tech support and they told me the S/510 would be more than enough....as did my dealer. So far quite happy with the end results.
Best of luck in your search.