I've been following the ongoing International Chopin Piano Competition and some contestants play on either a Steinway (there are actually two different Steinways to choose from) or a Yamaha CFX. To my ears there's apparent difference in the timbre and I do believe there is such a thing as sound signature. I can't say which one is better, they both sound absolutely fantastic. But if I have to describe them:

The Steinway seems to have more coherent sound, one that doesn't have a drastic contrast between fff and ppp but is a very smooth and gradual, this also applies to how the sound evolves in time from attack to sustain. And finally it also applies to the frequency spectrum (for a lack of more suitable words to describe that), very balanced sound with no frequencies sticking out.
The Yamaha CFX offers a very contrasting change between a bright and punchy fortissimo and a velvety/dark pianissimo. Then, the attacks are bright and clean, bell-like (but not in a bad way, quite on the contrary!) with the spectrum being almost two-timbral where you hear high overtones first and then they are withdrawn to reveal an entirely different base sound which is full and rich but dark and this spectrum seems to evolve. Absolute magic!

I think these are unquestionably the two best pianos in the world 🙂 Yeah, yeah, I know no two D-s are the same, and no two CFX-es. But I still believe in that signature thing since I've heard many D-s and many CFX-es and they confirm this sound signature theory.

For a comparison.

The Yamaha CFX

The Steinway

(I've linked to 7:02 in the second video, so that you can compare the same work, the Polonaise-Fantaisie).

What do you think?

I still vote for the Yamaha CFX although I would be hard-pressed if I had to prefer one over the other based only on listening to them on YouTube.