Hello Stijn and others,
you may want to have a look at modplug's source code (load_j2b.cpp) for a deeper insight into the J2B format. Everything is working correctly (apart from restart position, I have absolutely no clue where this could be hidden in the file, apparently it's hardcoded?) and I hope that there are at least a few helpful code comments in there. This is certainly more helpful than Dr. Eggman's findings (no offence).