REL S510 or S812

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.

I own 3 older RELs (Q series) bought used for  around 200 bucks each...one is in my video system. They all work perfectly, and although they're not under warranty (and REL won't repair them) they're so inexpensive that they've more than paid their way over close to a decade. 

I agree with hilde45 and audiorusty.

At -6dB even if you purchased five more of any REL "Sub-Bass" systems they all roll off the lower frequencies so dramatically so as not to excite most rooms bass modes. While this allows for room placement flexibility its at the expense of the subtle sub sonic extra low frequencies found in most recordings as well as broadcast and film.

Despite the speaker manufacturers rated low frequency your description suggests they’re positioned in the rooms null. No amount of equalization or optimization can overcome a null along with the huge amount of watts that would be needed to meet their rated output.

Return or sell your -6dB Sub-Bass speaker and purchase one or two actual -3dB subwoofers. It's very important to locate your rooms standing wave bass modes to position the subs in. Using the crawl (or walk) test while playing a low frequency test cadence, long (Blue Jean, Monoprice) interconnects and adjust to your taste.

@sandrodg73 

We have 4 S-510s in our main floor family room. They are fantastic with movies and music ! I recommend getting 2 eventually. Good luck!