What equipment should I invest in next?

BTW-love the match of the J2 with the GRs!
Although you would also have a near perfect amp for Quad 57s.
The current best are Electrostatic Solutions (ESS). They use thicker Mylar and get bass out of the Quads that are unique. If you sold your GRs you would be significantly there for the ESS at $6500.
I lived, joyfully, with 57s for ten years and I miss them most every day. If you decide to Call Robin at Robyatt Audio and get a pair you will thank me. To this day, the best midrange in Audio! You will be floored by the sound of a violin, etc., etc.

Thanks all!  Lots to think about and digest.  I’ll ask follow ups as I do research and try out different things. 
BTW, although I appreciate the ESL suggestion, I’m not investing in any new speakers for another decade or so!  But I’ll still check them when the pandemic is over. 
BTW, got my second shot today. So am officially invincible now. 

Is your room optimized? Tonality even throughout? Clear, articulate, big-enough soundstage? Highs smooth? Bass taut? If any of these things are not true, then the thing to do is consider that a big part of the sound is due to your room. It's not sexy or fun to advise someone with great gear not to get other great gear or fancy tweaks or cables, but it's really worth considering if you might benefit from traps, absorbers, and/or diffusors.

@ssmaudio, 
 I'll echo the chorus, you have put together a very nice audio system. 
I bet the First Watt J2-Tannoy Kensington GR pairing is special. Agree with you, no need to change your speakers.

You're headed in the right direction in  considering improving AC power quality and addressing vibration and resonance management.  You simply can't go wrong with either step.
Best of luck, 
Charles 

@charles1dad and @mglik Thank you! I really do like the First Watt and the Tannoy. They really work well together. The FW really does have an analogue-ish sound with very little coloring. And the Tannoys really have the same quality plus the concentric drivers have a wide sweet spot and very good clarity. Perfect for classical and jazz. I do have an REL sub as well which complements them well.

This is my first real audiophile system and I didn’t want to spend a ton of money and am very happy with it. It’s the first time I have my own listening room (kids etc. till now that limited my disposable income and space). Although the system probably cost more than my son’s annual expenditure at Juilliard.

@hilde45, I do have Acoustic Geometry curve diffusers at reflection points and panel absorbers at strategic places in my room. I haven’t done exact measurements—I’m not an expert on that, but it’s probably good enough for government work.