Hi Dnj,
On the DGX, the OTS voices are fixed. For each internal style, voice 000 is the OTS voice.
There are two drawbacks : 1) external styles don't have an OTS voice on the DGX650/660.
2) It is not possible to modify the OTS voices for the internal styles.
The workaround is to use registration memories. Once you've chosen a style either internal or external, choose the voice you want to use with it, then all the other settings (tempo, effects etc.) and save everything in a registration memory slot. When you want to use these settings, simply recall that registration.
Since there is no way to label these registrations in the keyboard itself, it is best to write down the bank number and the slot number as well as the name of the song for which the settings are made.
Regards,
Vinciane