The two Live! grand piano sounds are really superb, to my ears. Also, I have to say that EQ settings greater than 2 are definitely too bright when heard through the onboard speakers, IMO.
The comment I've heard in the past is that Yamaha pianos are too BRIGHT, compared to Roland and Korg. I haven't heard the muddy comment.
There is a tiny lag -- you'd measure it in milliseconds -- on a grand piano, compared to an upright. It comes from the fact that the sounding board isn't staring you right in the face, as it does on an upright. I can hear this on the YPG-625 -- a bit of realism in their sample that doesn't bother me at all. Might you be reacting to this?
Make sure both speakers are working, as most of the low sound comes from the left speaker. Also try different volume settings. The acoustics in your practice room might also be a factor. Try listening through headphones. Also make sure the keyboard has exclusive use of one electrical outlet. The manual warns that failure to do so could harm sound quality. I didn't find that to be true in my apartment, but the quality of electricity varies in different cities.
But when all is said and done, if you don't like it you don't like it. Options? The store where I bought my 625 has a policy that you can return a keyboard within 30 days for full store credit. This means if I hated the YPG, I could return it and have them order a Roland FP-2 or whatever, and just pay the difference. Wouldn't hurt to ask.
Since this is apparantly your first post here, welcome to the forum!