It does seem to sound slightly more resonant or "tingly" than the F# and A# keys, but to me it doesn't really sound like it's got a problem per se.
Some keyboards use the same sound sample for a given range of keys, but alter the frequency of the sample for the different notes, which can sometimes give a specific note some odd-sounding overtones as compared to the other notes of the chromatic scale.
I'm not sure how the DGX works in that respect, but another factor is that the notes of a piano are normally tuned according to the 12-tone equal temperament (or "12TET") tuning system, which doesn't have pure harmonic relationships between the notes of the chromatic scale the way that "just intonation" tuning does, so that can sometimes cause certain intervals to sound a bit "off" to some people's ears-- for example, a perfect fifth in the just intonation tuning system is often said to have a "sweeter" tone than a fifth in 12TET, because its frequency has a nice 3:2 harmonic relationship to the root note.
EDIT: Have you tried using one of the other Acoustic Piano voices for comparison? Also, you might try experimenting with different Reverb Type and Chorus Type settings, as well as the Reverb Depth and Chorus Depth settings of the voice.