Basicly Roland = GS format.
Most of roland gear supports GM but you will have to switch to GM as the default start up = GS mode.
I don't have an XP 50 but several other Roland boards.
To avoid your problem I would recommand to change All your midi's into GS midifiles.
The quickest way to do this and with the best control on all patches used is done on a PC with a sequencer like Cakewalk/Sonar.
As there are downloadable instrument patches for your XP50 for usage with Cakewalk/Sonar.
Fred
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Keyboards/Sound Units: Kurzweil 2600S, Roland VR-760, Acces Virus C, Roland G-800, Akai AX60, Minimoog, Machine Drum, Roland R8-M, mediastation x-76