Sub Confusion

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. 

@thebingster IMO that last Hz matters more than it appears. Those measurements  are at -6db. In reality, the S/812 is likely more like 24hz at -3db and the S/510 is probably 29hz at -3db. Both are not great measurements compared to many subs that dig much deeper, but Rel is known more for its speed than its ability to dig deep (aside from its reference products).