What equipment should I invest in next?

for any useful advice, op, you should carefully and specifically articulate what are the perceived deficiencies of your current system, or, said differently, what traits you would like further improved

otherwise you will get just random ideas on how to piss away money based on other people’s experience and systems, not yours

as you know, your system is already first rate, so to get real improvements require careful thought and consideration

you may also want to post a picture of your room and setup

Ssmaudio, don't spend money on those things, they will add nothing.
You are a violinist. You really need to hear a pair of electrostatic speakers
like Sound Labs 545s or Sanders Sound Systems Model 10s. They will send chills up your spine. I love string quartets. I can listen to them for hours. They may have been the singular reason I fell in love with ESLs.
No other speaker I have heard puts the musicians in the room with you like these speakers. No other instrument has the complexity of tone and texture that a violin has. With ESLs you can easily pick out the differences in violins. As you know there are good ones then there are great ones. Some get worse as they age others get better. Save up and get a pair. I promise you will never look back.

Another good one, in addition to Townshend Podiums, since you have both digital and analog try some TA-102 fO.q tape. Find it on eBay for the best price. I got this from another guy here who uses it all over his turntable. Most vibration damping materials lower the noise floor but tend to suck the life and dynamics out of the instruments. This tape does not rob any dynamics but does remove a layer of detail obscuring grunge that has the counter-intuitive result of improving presence. The improvement with strings and wind instruments is impressive!  

Using this tape is a little more labor intensive because the stuff has to be cut to fit but it is super effective. I started with a piece just 1" by about 1/4" on my tone arm and even that small amount was noticeable. 

Study my system pics you will see I ran a strip along the underside of the arm tube, and underneath the arm base. Basically I keep it discrete yet effective. You would never see it in person unless you really go looking for it. The best is remove the cartridge and apply the tape to the head shell. Has to be cut and trimmed to fit. But when done holy smokes! Groove noise vanishes! Details pop out everywhere! From 1" of tape!  

Can also be used inside your DAC, transport, phono stage. Great around tubes. $50 for tape seems expensive until you use it all and hear how much improvement it makes, then it seems cheap for what you get.

+1 on the Podiums although you could also get great results with Stillpoints under your Tannoys. Think the$125 Ultra Minis would be ok.  Also agree that you have a really good system especially for a modest price!
A super strong recommendation is to get  a Silent Running Audio VR platform for your TT. They are custom made for your exact TT and are $1500. I got one for my TT and the improvement is unbelievable.
SRA makes these things for Medical and Military. Their upper line are the Ohio Class and Virginia Class since their technology is used in nuclear submarines! About the best $1500 I ever spent.

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.