I have a similar problem with my Motif 6 and the powered Wharfdale box I am using with a 15 inch speaker and horn. The Motif acoustic piano has a nasty upper mid EQ characteristic that is very noticeable in the higher range. I didn't notice it nearly so much when playing through headphones but now I know it's there I can hear it now even then. I am thinking that the horn on my powered box tends to emphasize that frequency range. After reading your post I wonder now if it will always be present to some degree with any speaker system.

But I love my Motif as well. It is just certain patches in the upper ranges that display this characteristic. I try to avoid playing too much in that particular octave but that is restrictive.