Davt: I was thinking of making the same move -- can't believe there isn't someone out there who has experience with these two. Where is everybody?
I just replaced my 20 year old Koetsu Black with a new Koetsu Black. My turntable dealer (who tends to prefer Lynn and Lyra cartridges) remarked, after installing the cartridge on my Linn, "You've now got that Koetsu technicolor sound."
I don't know--are Koetsus "technicolor?" If so, I'll take two, please. I just love it. I think the review in Stereophile by Art Dudley from last July describes the sound very well, and would refer you to that. I would also say this: In the old days, my impression was that Koetsus weren't the greatest trackers in the world. I've heard since that this is not at all true, and my experience with the new Koetsu on tracking has been terrific.
I'm happy as a clam, FWIW. A dealer I know who sells both the Koetsus and Shelters is very fond of the Shelters and speaks highly of them, but uses Koetsus at home and in the premium demo system I heard.
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.