The DB50XG is only 100% compatible with the MU10XG and SW60XG.
I've tried to find this out.. but have only come across hearsay. Is there anywhere authoritative that says this?
The Yamaha manuals for the DB50XG and MU10XG have been PDFed and OCRed. Search them for QS300. If you search other XG manuals, you will not come across any mention of the QS300. I'm not why there is no midi implementation in the SW60XGs manual, but that does have the mode.