I haven't tried out any of the DGX models, but I believe the DGX-670 has a full implementation of XG, versus the XGlite implementation that (I think) was used in previous models. That means there should be more options for tinkering with the voices than merely adjusting the Master Equalizer.
For instance, doesn't the "Piano Room" feature that's included on some of the previous models have an option for how the "lid" is adjusted? (I'm not sure whether that's the right term, but my mind's gone blank just now.) And there are voice-related settings for things like the cutoff frequency for the high-pass filter and the amount of harmonic resonance. If I remember correctly, XG even has settings for additional filter types-- for example, low-pass and band-pass. Plus, there are the reverb and chorus types, as well as the reverb and chorus depths.
Also, you can combine two voices, so you can layer a second acoustic piano voice or an electric piano voice with the default Grand Piano Voice if you wish, and adjust its voice parameters separately from the main voice's parameters. You could even layer the same voice with itself and adjust the second layer differently than the first. Typically the second layer would be shifted up or down an octave, but it doesn't have to be.
Anyway, there are a lot of options for fiddling with a voice if there's something about it that doesn't sound the way you want it to. And the acoustic piano is generally one of the instrument sounds that musicians are the most picky about. I think that's why any electronic keyboard that includes an acoustic piano sound will typically have half a dozen to several dozen different acoustic piano voices to choose from. In any case, it seems like just about every musician I've ever seen posting in an online keyboard forum has been rather critical of the default acoustic piano sound on whatever new make and model of keyboard they've just upgraded to, and they tend to spend a good deal of time trying out the available piano sounds and fiddling with parameters to find something that sounds "just right" to their ears. And a lot of musicians will happily share the specific voice settings which they feel produce the "best" piano sound.
