I do not have a direct experience with these cartridges, but I have heard the Koetsu Black on a friend's Clear Aduio Champion II. I've been using the Shelter 901 for about 4 years.
Although the Black has that legendary Koetsu midrange, I thought it lack the dynamics and extension on both ends. Per discussions with many other Koetsu (black to Urushi to Rosewood Sig) owners in our local audiophile group, once you move up the line, you will get better results at the dynamics and bass/treble extensions.
My friend ended up trading his Black in for a Shelter 901 and confirmed that indeed the midrange sounded drier than the Koetsu black, but he liked what he gained in the bass/treble extension and felt that the Shelter 901 had a more balanced sound overall.
Shelter 501 Mk II has a very different sound than the 901. It is also known for its midrange sweetness, but as I stated before, I have not heard them side-by-side so I can not say whether the 501 Mk II has the same midrange magic as the Black. But then, we are comparing a $1600 cartridge (Koetsu Black) to a $850 cartridge (Shelter 501 Mk II)
I think some of the comparisons stated above are not fair comparisons either. The Koetsu Rosewood Standard lists for $2,600. I would hope that it is better than the $1,500 Shelter 901. I think a more fair comparison for the Rosewood would be the similarly priced 90X. A friend of mine had tried several Koetsus and both the 901 and the 90X. He described the 90X as a more refined 901 with the midrange of a 501 Mk II.
FrankC
