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

Its Fantastic that you are happy with your system. Rogue Audio makes great products and has excellent customer service as well. Very friendly for Tube Rolling - NOS Siemens are also some thing to consider along with the previous suggestions of Brimar, RCA and Telefunkens - ECC83, CV4003 - Brent Jesse is a great resource to check out for those products.  As a Rogue Amp owner I  have found that the more power I supply them (Power Delivery - PC) the better they sound!!!

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. There is a significant difference between the two models. I believe due to the frequency response of the B&Ws the T9i may just fall short of covering the range. You may already know this but the Hi Res Neutrik connection is the way to go. Rel sells them, but so does Analysis Plus and Synergistic Research. 

Continued Enjoyment of your System...

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IMHOyou  can not go wrong with another subwoofer no matter what else you plan on doing.

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".