Hello Larry,
Unfortunately, to achieve what you want to do, you really need a register memory slot.
When you power the DGX-650 off, Style Volume, Song Volume and Metronome Volume will remain into memory, but not the Main, Dual and Split voices volume.
I'd suggest you use Bank 1 Memory Slot 1 to store the Natural! Grand Piano voice at the appropriate volume, since this is most probably a setting that you will use again and again.
Regards,
Vinciane