Koetsu-like Cartridge recommendations

The Koetsu "Sound" is more or less unique based on its
Midrange Priority :-)
Maybe the Lyra Kleos is something for you. When I had it and compared
it to some Koetsus...well...probably it will give you better results based
on its high output with your existing Preamp.
On the other side, when you really love your Onyx, send it to Soundsmith
for a better Diamond, probably he can solve what you miss.

Can you demo a new Onyx? The recent carts have some changes in the design and the sound: tighter, without losing the lifelike timbre. Make sure you have an excellent transport. Koetsu has been designed on turntables, that can do rhythm, and on those, where this is not a priority, one could have some issues. The tone arm should be chosen carefully, Onyx is a low compliance cart.

I love my Benz LPS....

Yes, as Emil_f mentions, they have to be on the right setup. My Jade Platinum was not a good match for my Basis 2500, and Vector 3 combo. I think a stone body Koetsu requires a commitment to a specific type of setup. There is certainly a fair amount of info out there on some good choices. I am sure it would help, if you mentioned your setup.

Ken Kessler once said the Transfiguration Temper V has very similar bass to the Koetsu. Never heard a Koetsu in my system, but I do have a Temper V and it is exxcellent. The Shelter 501 is a warmer cartridge - so I guess an expensive Shelter may be a good alternative to a Koetsu.