I own a Koetsu-rebuilt Onyx Signature (original Alnico magnets), Koetsu-rebuilt Onyx Platinum, and Coralstone. All 3 styli are still in early life (but well broken in); good for comparison. I had the Sig - an original Sugano Sr. build - rebuilt because its original stylus was very worn (inner groove distortion, marginal tracking), even though it still sounded great. I instructed Koetsu to keep the original magnets. These magnets have the added benefit of fully DOUBLE the output signal vs. Platinums. That really helps keep down the noise floor.
The Signature has become my all-time favorite cartridge. It’s more musical and somehow also more neutral than the Platinums. It renders a better 3D soundfield with greater depth. And dynamics are a little more explosive. The magic Koetsu midrange/vocals are there in full force.
I also love the Platinums; they have a creamier, smoother spin on the sound that seems more refined at first. But at least for now, I love the Signature even more.
At first the Signature was a little bit disappointing after rebuild; it was just a touch dry and soulless for a Koetsu (still a Koetsu though), but it broke in beautifully and now sounds like it did before the rebuild, but without the worn stylus issues.
It’s of course possible my preference is due to the peculiarities of my setup & gear. I also listen to quite a bit of hard/alternative rock and heavy metal, which may affect preference as well (the Signature is brilliant for this genre). In a few weeks I should be trying out a Fidelity Research FR64S against my current Graham Phantom Supreme. I also have a Shelter Harmony on a Clearaudio Universal, that I really enjoy in the rotation to take some hours off the Koetsus.
If you do plan to source an Onyx Signature, please only have it rebuilt by Koetsu Japan! I cringe every time I see a beautiful Koetsu thoughtlessly affixed with that ridiculous SoundSmith ruby cantilever. You frequently see these for sale here with "under 20 hours" and the ubiquitous claim "it sounds better than stock"...yeah, right! There is also rumor that Koetsu Japan will reject any Koetsu modded by a 3rd party, though I cannot confirm this myself.