Okay, so I know there is no such thing as a perfect piano and that taste varies a lot between people. Here's my experience so far, so if you're up for it, let's see if we can share experiences w r t how to make the MOXF have a piano sound that sounds even better than the presets.
Prerequisites - I installed the "Ultimate piano" collection (free from Yamaha) so I could try out a few more pianos. I have not bought a 3rd party piano sound (yet).
Here's the first thing to note: EQ. The three band EQ on the knobs is not what you want to check out - most piano patches have a 5-band EQ on insertion effect A and is usually tweaked to what Yamaha thinks sounds best. Here's where you want to fiddle, rather than the Low/Mid/MidF/High knobs you have available first.
Second thing to note: Reverb. All of the "Ultimate piano" sounds (I think) come with no reverb preset at all (!), so if you want to have reverb, enabling system effects is not enough, you have to edit the sound to add a reverb.
So far, the standard grand piano "Full Concert Grand" is the piano sound I like the best. At soft velocities it sounds really great, nothing to complain about. However, at higher velocities I experience a "ringing" in the attack of (at least) notes B, C, C# (where C is middle C). The ringing suddenly disappears at D and then comes back at F, F# and G (again, just above middle C). I've tried to identify it and I believe the ringing is somewhere around 7 - 8 kHz. I've tried to edit it away but so far I failed. I could turn the EQ down, but then I would have to do it so much that it hurts the rest of the notes and the richness of the entire sound.
Ideally, I would want it a little brighter as well - i e, something between the "XF brite piano" and the Full concert grand, but without the ringing described above. I've used the 5-band EQ to brighten the "Full concert grand" a bit, but unfortunately that increases the ringing as well.
I've also tried Edit -> Element 4 and 6 -> Filter -> Scale to turn the cutoff frequency down for those notes, combined with changing from LPF18 to LPF24 it does seem to help a bit.