By default, the Notion Mixer assigns only a single stereo channel to a grand Piano staff. As a result, chords in the left hand are much heavier than single lines in the right, for example, and mask other notes in the score.
The solution is to send the bass and treble staves each to their own dedicated channels. Sibelius does this by default, as do several other notation packages I have used. While it's possible to emulate this using individual staves instead of a grand piano staff, having a piano staff configured this way by default would automatically make playback more realistic.
And should be quite easy to implement.
If you like this idea, please vote it up. Thanks.