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

You guys, I am absolutely enjoying my stereo - as stated above, I love it! I’ve been living with it as-is for about a year now and still love it. I listen to it every day. 
I recognize that just going to floor standing from stand mounts would change things (likely for the better) but maybe not? I’ve read that the 804d3 is actually less detailed because of the two in cabinet woofers vs. the 805d3. I wonder if trying a second Rel sub is a better route? I agree - I just need to experiment a bit. Or maybe I go back to an all tube phono stage? So many options for sure. 

If you're happy with what you have, that's awesome!  If you want to take it to another level, add another sub (or 2 or 3).  You'll get a lot more bang for your buck out of that than buying expensive cables.  Not that they can't help, but whatever improvements (if any) you get from cables will be subtle, at least to most people.  Adding more subwoofers, especially if you have four of them will be more than a subtle difference.

Most audiophiles are habitually discontented, or only happy when engaged in the next hunt, but not when it's over.  If you're truly content, don't for heaven's sake rock the boat.

Four subwoofers is definitely not an option for me. I have one currently, which I’ve hidden from sight. I actually don’t like what “gear” or stereo equipment. I love minimal, clean layout and interfaces. I love analog. 

There’s a lot of unmapped territory out there regarding RFI/EMI and how to deal with it. It’s like Chicken-man. He’s everywhere! He’s everywhere! 🐔 🐔 🐔 It’s generated by electronics, room purifiers and other appliances, Coming through the house AC, not to mention all the TV, radio and cellphone transmission coming right on through the windows! Hel-loo!