Sqk,

What happens if you try and use those XG/GM/GM2 voices via a USB connected to the DGX-670? Sounds odd that a Yamaha midi keyboard won't load GM voices.

I assume you already tried ?
GM/XG/GM2 Voices

FYI. Legacy voices are factory installed in the SX-900, and available for a main voice selection. And the above GM/XG/GM2 voices also load and play fine in the user (USB) voice selection.