I had tried to understand the Koetsu products over the last month, using 8 different Koetsu Cartridges and different tonearms.
Unfortunately, Koetsu is not simple. Means you cannot walk from a cheaper Koetsu to a more expensive Koetsu in the same environment and expect a "better" sound.
You just will get a "different" sound, you may like, you may like not.
I see the family of wood bodies, the family of Urushi bodies and the family of stone bodies as families of acoustic art in its own.
Listening to an Urushi Vermillion can be the "best" Koetsu experience , being better than any stone body in a given environment.
Same with the stone bodies, an Onyx platinum is so unique and different to Urushi, that it is difficult to love both....
The Coralstone Platinum is a kind of compromise between Onyx and Urushi, being sonically between both characteristics.
But.., you have the chance to fine-tune the Koetsu , using different headshells, headshell leads and so on.
The Urushi will play also on medium weight Tonearms, the stone bodies will prefer more heavy ones (just my personal experience)
So for me every single Koetsu is an individual peace of art. (I had three Onyx available for an comparison, the weight difference was significant, so a "real" comparison not possible, as the weight will change the effective mass of the tonearm/cartridge system. Stone or wood are natural , so the weight on constant dimensions will differ.
The optimization need to be done for the individual Koetsu in the system.
If done right, I love the performance over many others.
But it is also easy to get a kind of boring, not involving set up.
I prefer the FR 64s or FR 66s, my newest Tonearm with a good Koetsu harmony is the old Ikeda 407 , which has a solid steel armwand similar to FR66s.
This Ikeda 407 version is matt metal design, the later ones were (are) chrome or gold. The newer Version is fine, but the older Version outperforms with Koetsu.
It seems, that specially the stone bodies even like a heavy tonearm base. So using the Fidelity Research N-60 or B-60 heavy nuts, or even the Micro Seiki AST-10 do increase the performance of the FR and Ikeda tonearms with Koetsu .
Unfortunately I had so far no chance to listen to the Koetsu SAR or Axiom combination.