REL S510 or S812

Im very happy with the 812.  Admittedly, I don't have much to compare it to as it is my first higher end sub, but the high level connection works as advertised.  Separate volume controls for LFE and high-level is really nice.  For HT use, I'm sure you could get more boom elsewhere, but I kept my REL HT1205 and use both for movies....sometimes music too.  

Can you feel the bones in your ears rattle?

Is your SVS 3000 a sealed enclosure or ported?  

I have two S/510.  They have more than enough output for accurate music reproduction, but they aren't quite enough for loud explosions, etc for home theatre.  I use a third SVS 12" ported sub for that.  The REL S Series is really more about accuracy than output volume.

I've never tried to crank the S/510 beyond what's needed for accuracy.  I would imagine that with the right type of music and cranked up, they would get pretty loud. 

SVS is sealed. Great sub.

Leaning toward the 812 at least I’d say.

In the S series, I'd say you would want the 812 given your requirements.  Two are better than one for two channel, but that gets kinda pricey. 

My SVS sub is a PB12-nsd. It's only a 12 inch, but it's pretty big. It will shake the house. It's ported.