Sub Confusion

As mentioned, the 212/sx is designed to run in tandem with your main speakers (no high pass).

Generally, stereo subs would be preferable. That said, i have a single 212/se in my system and don't want for anything below 80hz, so there is always more than one path to satisfaction.

I agree with just about everyone’s posts. That said, 80Hz high pass might be a bit high for your speakers considering they can dig down to 26Hz. If your goal with subs is to clear up the frequency spectrum and deepen the soundstage, you likely want them performing largely below audible frequencies and will want two or more in the room.

Even without a high pass, you will be surprised what two good subs crossed over at 65Hz or lower will do to the rest of the spectrum and to the sound stage. Should you still want to combine this with a high pass for your speakers, I suggest looking first at a 60Hz high pass as 80Hz may be too high.

While the Rel 212 is a wonderful sub, it is a lot of a single sub for that room. The others you mention though do not dig deep enough. I would probably opt for two 812’s, and if that’s too much money, the SVS SB-4000 is a worthy competitor at a fraction of the price and offers better integration with its mobile app.

In full disclosure I run a hifi business but do not carry Rel or SVS. 

"SVS SB-4000 is a worthy competitor at a fraction of the price and offers better integration with its mobile app."

Ummmm do SVS subs connect via the main amps outputs? I dont think so. the above statement will never be true, just because theres an APP? Thats not how subs integrate.

The REL subs connect through the main amplifiers outputs just like the main speakers for true integration and you dont do this using Hpass filters either!

If you need more than one sub your using the wrong subwoofer.

And to answer the OP's question YES the S510 will be plenty for your room size. I would buy it in a heartbeat. And make sure you set it up as REL reccomends for best bass response and musicality......YES REL's do more than produce bass they are musical and when tuned in properly they make the main speakers come alive even more and allow them to play with more detail and ease. REL subs sound beautiful.

Thanks

Matt M

Thanks to all for the valuable input. When I said I didn't need more bass, I meant what blisshifi said is exactly what I meant- "to clear up the frequency spectrum and deepen the soundstage". However, blisshifi, mentioned that other than the S812, the others don't dig deep enough. I know there's a difference between the S812 and S510; however they are both -6db, with the S812 at 19hz vs the S510 at 20hz. Given that, and based on my room size, wouldn't it be wise to start with a S510 as suggested by Matt M (setting the crossover at 65Hz)? 

Vandersteen sub3 w high pass filters if your integrated has pre out / power in. Better bass and mids….especially the mids….