et tu Koetsu?

I've heard that the Rosewood Platinum is less musical than the Signature. Any thoughts?

Quite likely that the phono stage could not handle the 0.2mv output from the Platinum.

Having owned more than half a dozen Koetsu:

1) Important to load at 100 ohm.
2) Black is the 'classic Koetsu' sound, a bit slow, rich. Bass is big, high-end is not fully extended. A 'Tube' like sound.
3) Rosewood Sig (RS) is the "real Koetsu' cartridge. Even the original late 80's/90's RS properly loaded, in today's electronics, sounds conetemporary, in the same league as VDH, Lyra, etc. But overall, I detected a slight hard-edge which is gone after moving up to:
4) Urushi - one of the most musical and high end Koetsu. I had the original Sugano Sr. version, and 2 years ago, refurbished to current spec. Both spec has the strong musicality, but none of the slowness or comnpromises as in RS or Black. It is very fast, goes down deep and goes up high. If I would say that the Urushi sounds like 'Dom Perignon', then the Helikon sounds like Evian mineral water. This is not to say Helikon is a bad cartridge, but it is very neutral, and pure as water, while Urushi extracts the full flavor of the music.
5) Onyx - Onyx compared to Urushi, it has the same tonal and refinement (having the same coil and stylus structure), but it has different dynamics. This cartridge has extremely linear power, from ppp to FFF, but it however does not have the 'sparkle' effect like the Urushi. A gloriously Hi Fi spectacular, but not as fluid and musical compared to Urushi, in my opinion.
6) Platinum version - different magnet, different coil and can be had with Rosewood, Urushi, or Onyx. The Platinum would retain the same characteristic as the non-Platinum counterparts, what Platinum improves upon is the dynamics and the realism of the playback (but remember, the drop in voltage makes the phono stage setup extremely tough, so the improvement may not be fully realized). It is very difficult to describe the sound of Platinum in words, but basically, you would hear a beautiful violin through the non-Platinum, but with Platinum, you hear more of the shape of the violin box, and the radiation energy of the sub-tonal pressure from the wood beyond the string sound. In a bass drum, Platinum can project the pressure of the drum that makes the music more 'live'.

Hope above helps.

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.