REL S510 or S812

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!

Thanks again everyone 

I just purchased a REL S/510.  It replaces a 20 year old Velodyne HT sub.  At first I was going to purchase the HT/1510.  My local audio guy convinced me to get the S/510 instead.  He knows my system inside/out and more importantly, the characteristics of my 14 x 12 listening room.

My listening experience is 80% music and 20% tv and movies.  My main speakers are B&W 804s.  The S/510 works seamlessly with all my source components: Denon DCD A110 SACD/CD player, innuos Zenith mk3 streamer + innuos Phoenix USB, and Bricasti M3 DAC, Modwright Oppo 105, and EAT C-Sharp turntable/Sutherland phono pre amp.  

Rock, jazz, classical, electronic, etc., all sound fantastic to my ears.  My audio guy is a graduate of the Berklee College of Music, and he knows how live music should sound.  I consider myself a lucky guy because he has my system tuned perfectly, IMO.
 

If you are more of a movie guy, you may be better served with the HT/1510.  The S/510 does not produce slamming bass that will make your teeth chatter.  It is smoothly integrated with my B&W 804s, and that combination brings tight/punchy/fast/full bass to the music I listen to.

Good luck!