Please get and post the device info from the XP64 system, like this...
for (int a=1; BASS_GetDeviceInfo(a, &info); a++)
printf("%d: [%s] [%s] [%s]\n", a, info.name, info.driver, info.driver+strlen(info.driver)+1);
If the system has a single soundcard, please also try connecting a USB soundcard for additional data/comparison. Also, are you running the same DLL version on all of the test systems, ie. it's not just that the XP64 system is running an older DLL?