et tu Koetsu?

Yikes! Yellowness? What does that mean sonically? I have an Urushi with about 50 hours on it now and I play everything from Zep to Stooges to Ellington to Respighi to Ray Brown. Everything I throw at it sounds natural and uncolored. Trackability is first rate at 2.0 grams. Dynamics are powerful, plenty of air and the bass goes all the way down and is clear and not thumpy/lumpy at all. The mids are all there and are detailed and sweet, not laid back in the least. I used to use a Glider L2 but I won't be going back anytime soon. This is one cartridge I can easily live with on a single arm (JMW 10.5).

Interesting analogy of transforming Mahler into Janacek (why not transform the Halle Orch/Barbirolli into the Czech Phil/Neumann...?). What verybigamp is probably hinting at is the analogy "transform the Deep Purple into the Deep Purple + London S O"... Older Koetsu had a beautiful sound at the expense of some db loss at freq extremes. Not that the fact ever bugged me when I used to have a 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.