I just bought a new px350 couple of days ago and have the uneven volume problem - especially in F and G above middle C. Guess I'll have looked at while my warranty is still fresh. Seems also that the "bright" component is too bright on those keys although it could just be volume/sensitivity. I set the touch control to off and all keys were way too loud. Set it to 3 and it was better, but it's only a temporary setting.
Update: I was learning to work around this problem, but then got the appropriate adapter to connect the piano to my stereo system. Problem disappeared. Every sound is great. Apparently the speaker has a exaggerated resonance with the keys F-G above middle C.