REL Vs SVS for 2 channel music listening

I have a REL S5 and couldn’t be happier with it. When I purchased my REL a few months back I asked similar questions as you and considered the SVS as well. In the end I learned that one of a sub’s most important responsibilities in sound reproduction is to create a sense of space. The REL’s drivers are very fast and integrate seamlessly with my PSB T3’s and help to create an enormous and well integrated sound-stage. I love what it does with 2 channel music as well as movies. REL's philosophy and approach is a little different than other speaker manufacturers. I'd suggest going on Youtube and searching under REL subwoofer. Find a video where John Hunter, CEO and Chief Designer at REL talks about the REL philosophy. There are several very good ones.

Good luck with your purchase.

In very broad terms the larger the driver,  the deeper it will go and the louder it will play without distorting. Conversely in very broad terms, the smaller the driver the more dextrous it will tend to be but will not go as deep. If you take a look at the entire REL line-up,  I believe they have 9 subwoofers total. 

Part of the REL philosophy is to find the subwoofer match for your speaker system and listening area. I'd suggest heading over to a shop that carries both SVS and REL and listen to them in a 2 channel system. 

Calloway... 
I have an S5 as well, and chose it over the 212 SE.
1. Recently I've been wondering how much I am missing with the S5 vs the 212 SE.
2. I'm thinking one of the upgrades this year is dual subs, similar to yours. 

Thank you in advance for any response... 

Calloway how do you you have your RELs connected through Neutrik? Which REL is connected to what speakers and where do you have them placed?