REL S510 or S812

I own 3 REL subs. Use them all primarily for music. S812 is fabulous and I have them paired with my Wilson Alexia V’s. I use the T9x for music in my office. 

I also recently purchased a REL S510 for my 2-channel system to supplement my PMC Fact 12 3-way floorstander speakers in my den which is 12' x 17' x 10'h. Does exac tly what oddiofyl says; except I have it crossed over at 40 Hz. Fills in wonderfully and creates broader soundstage. I also debated about going larger with the S812 but glad I didn't since the S510 was more than enough for my room and setup. You'll love it- looks great as an added bonus.

Paul at PS Audio is a big fan of Rel.

I've had a pair of REL S510 for two years that I use for both my HT set up and my 2-channel system. I'm a huge fan of REL -- I like that you're able to have HT and 2-channel connected at the same time each with their own adjustment. With my systems, the sound is spectacular. Like you, I debated between the S510 and and S812 and while I have no doubt the latter is even better than the 510, I determined that the smaller one was more than sufficient for my room size and system -- and that was a good value decision. The only other thing I'd say -- they're an incredible addition to a 2-channel system and if you can swing it, buy two of them. It really adds to the soundstage depth and breadth. Good luck.    

I own 2 rel 510s. Unless you have a very large room, the 510 is plenty. The volume is at about 25%. They add depth to the soundstage and the low mids are great. I use them in my ht as well. The biggest issue is that some music producers use too much bass and some low notes resonate in the room.