Looking for the best moving coil cart that is around $5K used

I don't want to write a treatise on analog v. digital. I have a Nantise/Lenco turntable with an Ikeda Kai cartridge, a 407 long tonearm, a Star Sound Platter Ground, and a H-Cat X12 current phono stage for analog.
  For digital I have a Surface touch screen control unit, and for digital, an AMS music server from Archiving Vinyl, an Avarii dac with JFET amplification. My digital sources are all double DSD initially on SACD.
  There are many differences. For one their sound stages differ. Digital is what the mikes receive and analog is what the recording engineer seeks and is more of what an audience hears. Second, there is more detail in digital, of instrument noises such as more harmonic as well as fingers sliding on strings. Finally, digital has highly defined locations of the sound stage and better high end.
  But analog is smoother, has more music hall or studio decay, and has more familiar instrument sounds with brass and drums not accurately reproduced.
  These observations come fro about ten instances where I have both analog and digital versions of recordings.
  There is no question that digital captures more detail and precise location of musicians and signers and that some of this is not music but it contributes to  realism.
  Vinyl, however, gives more hall decay and sounds familiar.

I read recently that the most important part of a phonograph system is the turntable, followed by the tonearm, folled lastly by the cart. It doesn't make any sense putting a $5K cart on a $400 TT/Arm. I would recommend selling your two tt setups and buying one new one of the best quality TT  and Arm possible and run it with the cart you have now.

handymann

At $5000 used, you have many wonderful phono cartridge options.  Nothing is perfect in high end, so chose something which best suits your preferences.  Try a used stone body Koetsu for sheer tonal character or perhaps VDH Colibri African Blackwood Mk.2 series if you cherish speed and dynamics.  See if you can borrow something first, if possible. 

On Audioasylum there's a NIB Kondo IOm cartridge listed for $5500. Went new for well over 10k.

The above post written by itsjustme is spot on.  I have a real world example that illustrates his point.  The turntable involved in this anecdote is a VPI Prime and when I first bought it, I was having trouble dialing it in.  Part of the problem was that I was comparing it with a Technics SL12OO.  Direct drive vs. belt and all that.  I bought a Phoenix Engineering Eagle PSU and Roadrunner Tachometer, which solved the speed difference between the two TTs.  One of my favorite records is Time Further Out by the Dave Brubeck Quartet, with Joe Morello on drums.  I am a drummer, Joe and I both studied with George Stone, and I worshipped Joe the way kids will.  So much so that I bought and still own a set of Ludwigs just like Joe used.  Well on Time Further Out, the recording of the drums is magnificent, so much so that on certain tracks I can hear the air passing through the air port on the top of the bass drum.  That is I can hear it with the Prime plugged into the Eagle PSU, or using the SL1200GAE.  That is because the speed is held consistent, which can be seen with the Roadrunner tach.  At the same time, the speed can be deliberately changed up or down on either TT without losing the ability to hear the air port.  But, if I remove the Eagle and the Prime is plugged directly into the wall, even though the bass drum tone does not change, the air port vanishes.  With tight control of the power supply flutter goes down and this can definitely be heard on a good TT.