[accession]

There lies your problem.

If you pull down the volume on a MIDI track, it'll do the same to everything running on the same MIDI channel connected to that MIDI port.

Your XP-50 has two modes:
- One for solo performances (responding to a single or any MIDI channel, and only playing one instrument sound at a time)
- One for full arrangements (using different instruments assigned to different MIID channels, playing back independently-yet-simultaneously).

You need to set it up for the latter. I've forgotten Roland's terminology. Dig out the manual and suss it out. Your want to set it up to function as a "multitimbral instrument".

Back in Pro Tools, use a different MIDI Channel for each instrument (eg. 16 channels, so up to 16 different instruments playing at a time).

If there's a specific rhythm channel (eg. Ch10), then you can create multiple MIDI tracks to assist with editing, though they'll all need to be sending out on MIDI Ch10. In this instance, the volume control will affect all tracks on the same MIDI Ch. To alter individual volumes in this situation, try the adjusting the velocity levels instead (see track view).

Good luck!

(You'll probably want to be able to recall patch names from Pro Tools too, right? Another lesson for another day. )