Seems then your system prefers the DirectSound driver, or it must hate xaudio2?Did the build just before the x64 changes work for you?
Just to reiterate, the problems you are experiencing are more likely due to me bypassing BASS as an output method and using my own output interfaces instead.
example MIDI Human.MID Warcraft II 047 for example ( Timpani ) missing
Human1GM.MID for example. In windows 7 i can hear all tracks. In windows xp the timpani 047 is not present and sweep pad 095 maybe. (tested with v3.19 and v3.17)