The BASS_ATTRIB_PAN attribute is a balance control. If you would like to have separate left and right volume controls (like with BASS_ASIO_ChannelSetVolume), it is possible to implement that by using a combination of BASS_ATTRIB_VOL and BASS_ATTRIB_PAN settings, like this:
BOOL SetLeftRightVol(DWORD handle, float left, float right)
{
float vol, pan;
if (left==right) {
vol=left;
pan=0;
} else if (left>right) {
vol=left;
pan=-1+(right/left);
} else {
vol=right;
pan=1-(left/right);
}
return BASS_ChannelSetAttribute(handle, BASS_ATTRIB_VOL, vol)
&& BASS_ChannelSetAttribute(handle, BASS_ATTRIB_PAN, pan);
}