It isn't possible to use device number -1 in a BASS_GetDeviceInfo call (BASS_ERROR_DEVICE will result). To get the name of the default device, you will first need to locate the default device by looking for the BASS_DEVICE_DEFAULT flag, something like this...
BASS_DEVICEINFO dinfo;
for (int dn=1; BASS_GetDeviceInfo(dn, &dinfo); dn++)
if (dinfo.flags&BASS_DEVICE_DEFAULT) { // found the default device
printf("%d: %s\n", dn, dinfo.name);
break;
}
Device number 1 will usually be the default device, but not always.