I haven't heard any old (father) Koetsus in a long time, but it's my impression (from my research) that the sons have done a good job of building upon what the father started. There is little doubt that the current Koetsu's are excellent. My experience listening in a well known, controlled environment is limited to the son's entry level Black cartidge (I think the father made a different Black model in his day). The son's Black model is a magical cartridge in the midrange, good in the highs and a bit rolled off on the low end (not bad, but not nearly as impressive on the low end as it is in the midrange.) I compared it to a Dynavector 17D; the two had different signatures from top to bottom and overall the K. Black was more natural and listenable, but the 17D had some definition and texture that stood above the Black in some respects. If I had to live with one, I'd take the Black (which is as it should be for rougly twice the price.)
After trying the K. Black I moved to a Shelter 901. While I wouldn't take anything away from the K. Black, the 901 was just a better cartridge all the way around. It matched the K. Black on midrange, including air. The Shelter was superior in terms of detailed and subtle highs, and it pretty much blew away the K. Black on the low end. The Shelter went lower, was tighter, and it provided a better tonal balance from top to bottom. The Shelter has great imaging, always sounds natural, and it tracks well. The Shelter 901 is one of the most impressive audio components I've heard in 30 years. The 901 brings not only tonal balance, definition, detail, air, and imaging, but it does so without ever causing fatigue. It is a marevelous balance of definition and musicalness.
From what I've read, the Shelter 501 II and possibly the 301 share many of the same qualities of the 901; if you are looking at cartridges in these price ranges, I'd consider the 501 II and the 301, along with the Dynavector 10X5 which wouldn't give you incredible detail, but it will give you decent definition, very good tonal balance, and strong musicality (without fatigue). If your budget allows, go for the the Shetler 901; and if you have a bigger budget people who have had both the 901 and the Shelter 90X say the 90X is somehow still another full step up, although it's hard to imagine what that would be after hearing the 901.
Having said all that, I don't want to take anything away from the Black or other Koetsus; it's not matter of whether they will sound excellent, it's just a matter of whether there might be another cartridge that will be as good or possibly even better for the same price or less. On the other hand, if your heart is set on Koetsu, get a Koetsu. I don't think anyone would be unhappy with a Koetsu - but if you can find a way, compare any entry level Koetsu with a Shelter 901 or any midrange or upper level Koetsu with a 90X. (To do a useful comparison, it should of course be in your system in your room. As with all components, cartridges must not only be good in and of themselves, they should have synergy with the rest of your system.) Whatever wins the comparison will give you a cartridge that will allow you to turn your attention to some other component, or maybe your room acoustics, or if you are lucky, you can just move on to listening to the music.
