Update needed

Your set up is pretty well matched, any one change will make a small difference, but not a huge one until all three are switched out. My opinion here, but I'd upgrade the turntable first, keeping mind the level of new cartridge you are aiming for, then the preamp. Normally I'd suggest the cartridge first, but I own a Raga P6 and Rega's are known for not being very adjustable, so you might end up with a cart that is not that much of an upgrade. Therefore, the turntable first, so you can upgrade to a much better cartridge, then the pre amp to get the most out of the new cart. That's what I would logically do if I couldn't do the whole thing at once.. 

IF you like Jazz, there are a heckofalotta great MONO LP’s out there.

I was convinced here to try a real mono cartridge. It is definitely better: a bit, some, a lot.

Thus, you want a tonearm with removable headshell, and an alternate MONO cartridge mounted, aligned, ready to switch. 

Removable headshell opens the possibility of having a MM stereo cartridge and a MC stereo cartridge, now or in the future.

A friend I met here saw/heard my setup (3 arms) and decided he wanted the potential for 2 tonearms specifically for the Mono Jazz LP’s he had and would be buying. We researched, he settled on a used Luxman PD-444 with two arms. It’s an elegant looking TT IMO, and the technology can compete with new equipment.

Point is, he was all set to buy a new TT, single arm, fixed cartridge until he saw the true ease of switching stereo/mono in a listening session.

btw, that Luxman has a built-in Junction box to select which tonearm goes to your phono, so you do not need more than a single input on your SUT or phono stage. I need a SUT with 3 inputs and PASS for MM.

here’s a nice one

used is always a bit of a risk, but I bet it works wonderfully. Chakster, our missing analog friend, had two of them in his system, that’s when I learned they existed.

Check for any used item: this seller takes PayPal, accepts returns, and use your best credit card to fund PayPal.

at the top of this page, click ’images’

https://www.google.com/search?q=luxman+pd-444&rlz=1C1VDKB_enUS1007US1007&sxsrf=ALiCzsbR5oDOsDcCBc1OYl6CL8hNaharIg:1655206346337&source=lnms&tbm=isch&sa=X&ved=2ahUKEwiBtLaS7Kz4AhUbjYkEHbvqDAgQ_AUoAXoECAEQAw&biw=1592&bih=815&dpr=1.5

NOTE: Dust Covers. Many TT’s that allow two or more arms do NOT come with Dust Covers. The Luxman design includes a hinged dust cover. The JVC Plinths CL-P1, CL-P2 (mine), and CL-P3 come with OEM lift off dust covers. They start on top of the Plinth, thus not too tall.

Custom dust covers for other TT’s can get quite large and unwieldly.

The Luxman is wide and shallow, 2nd arm on the left, fits certain locations.

The JVC Victor 2 arm Plinth CL-P2 like mine is less wide but deeper, 2nd arm in the back.

click ’images’ on this page

https://www.google.com/search?q=jvc+plinth+pl-2&tbm=isch&ved=2ahUKEwjbjdjQ7qz4AhVtGVkFHUKlBmcQ2-cCegQIABAA&oq=jvc+plinth+pl-2&gs_lcp=CgNpbWcQDDoECCMQJ1CmGFjfKWDVPWgAcAB4AIABUIgBnQSSAQE4mAEAoAEBqgELZ3dzLXdpei1pbWfAAQE&sclient=img&ei=ZXSoYpvHL-2y5NoPwsqauAY&bih=815&biw=1592