et tu Koetsu?

Your description of the Koetsu sound is the most accurate I've read and mirrors my experience, including the Helikon. I am using a Urishi Vermillion with an Aesthetix Io dual chasis signature. As good as the Urishi is, I am considering the Onyx Platinum, but have heard that it is cold sounding and slightly bright and that the Rosewood Signature Platinum is smoother with more resolution than the Urishi and not as cool as the Onyx Platinum.
Perhaps you could contrast these three cartridges in your own eloquent style. Also, I am not aware of a Urishi Platinum.

The Urushi Platinum will be my next Koetsu cartridge. The UP and some other models, such as the one with all-diamond cantilever is not available in US or Japanese retail market, but through people that knows Sugano, they can be obtained. In another word, if you know how to contact them, and pay the right money, they will make any combination of body, magnet, and stylus. But I believe the Ruby or Sapphire cantilevers are no longer available, only Diamond cantilevers can be had.

If you like Urushi, than you have no reason not to go for the UP. In fact, you can retip your Urushi upon its expiration with the Platinum magnet.

If you are looking at Onyx Platinum; an old Onyx, so long as it's the current wide body design, can be retipped with Platinum. In fact, older Onyx by Sugano Sr. looks more transparent, and new Onyx has more stone grain, and less transparent. I personally prefer the 'cooked pork fat' look of the original Onyx vs. 'white marble' look of the new Onyx.

My source tells me that the Urishi Platinum is not available at this time.

Very good, Extremephono. Thanks.

Thanks, EP. Rare is the person that has personal experience with that number of Koetsus and has the ability and willingness to provide desciptions that are meaningful and helpful.