My old Mix+ system was on a Power Mac 9600, sold the Mix cards on eBay while the getting was good, still got the 9600 around here somewhere. For SD2 files Rail Jon Rogut still has his SD2 to Wav converter for free
HERE.
PS. I recently found my old Audiomedia III card which will run on PTLE and I'm actually downloading PTLE 6.2.3, not sure if it works though as I can't find my LE authorization code. Might be able to convert his sessions with it. Also able to download PTLE 5.1.1 for OS9, the old Digidesign support pages are alive & well. Gonna search my backups and see if I can find the authorization codes for PTLE to check it out.