How good can it get, really? - my stereo sounds amazing!

You added a second Sub to your speaker set up, which I agree with (I have a pair of B2s) however you seem to feel they are a little off ? I will point out that Rel recommends the S/510, the next model up from the T9i as an appropriate match to your B&W 805s. 

Yes! I do feel that the low end is off! I think I’m going to upgrade to the S/510s. Thanks!

Why mess with a good thing?  If you are satisfied with what you have, why go looking for improvements? It isn't the equipment that you have. It's what your ears hear and whether you are satisfied with it or not. There IS no 'one size fits all". 

Recently, I faced the same dilemma.  I have a very good system. However, it is in my nature to always seek "better", even though by now I have taken mine beyond what most people on this forum have proposed as modest to major potential improvements to the original poster's system.   For example, the concrete floor of my AV room has been sawed to be physically separated from the rest of my house, then covered with acoustical matting and finally with carpeting; my walls and ceiling are of multilayer sheet rock; acoustical paneling was placed on the walls wherever necessary to minimize undesired reflections; and the room/system itself has been tuned via Dirac Live.  Not surprisingly, I have continuously upgraded my equipment (including power conditioning and cabling) for the past 15 years.

Increasingly, I have become aware that (i) the law of diminishing returns is a fact and (ii) the more complex one's audio system, the more interrelated -- and therefore sensitive to even what would seem to be a very modest adjustment -- is its performance, giving rise to the prospect of unintended consequences.  My final education, if I can call it that, came last month when I supposedly upgraded a power cord for the Ethernet switch in my system.  The result of this was a clearly demonstrable, and significant, degradation of the magnificent sound I had been enjoying. 

From this experience, I now am trying to listen exclusively to the music playing -- both to its content and how it sounds and to stop trying to analyze whether the reproduction is sufficiently "life-like".  This has been hard for me to do, and yet I intellectually recognize that, given my 73 year old ears, my system is already as good as I can hear.  

Damn that sounds you have an incredible set up! I doubt I’ll ever take things that far but good for you! That’s awesome. I love people who take things to the extreme. As I’ve stated I do love how my stereo sounds and I’m not really chasing a “live” performance sound, I just want it to sounds great! I feel I’m pretty good at enjoying the music and subjectively analyzing the sound quality at the same time. The sweet spot is when it sounds so damn good you forget to analyze and just enjoy. 

Keep an open mind. "EVERYTHING"- affects a systems sound! 
Don't get hung up on ANY "Specs"!
Take the time and put the work in to hear, "What (IS) possible".
 As in going to "AXPONA", and other trade shows. And do NOT be that person that is- "Just Looking", at the gear. Listen to it. Ask questions...
 Eventually you are going to find a set-up, OR several, that simply, "Blows You Away"!
 Find out why. Talk to the builders. And go from there.