I only use software synths, but I was thinking about your problem and I remembered that BIAB has a MIDI Monitor that you can use to analyze the MIDI codes being generated. I tried various volume settings in BIAB and looked at the resulting MIDI codes. Changes to the volume in BIAB had no effect on the MIDI codes being generated. There were also no codes being sent for tone, pan or reverb--Those are the other controls next to the volume control in BIAB. Therefore, I believe that controls are applied to the tone after it is returned to BIAB from the synth. If your hardware is setup so that the external sythn is output to an amplifier (I don't know if there is another way) then BIAB cannot process the sound afterward, and so you would have to create CC events, instead, to control these aspects of the sound.

Now, maybe someone who uses hardware synths may be able to tell you if there is a different way to setup the hardware, or if there is a way to force BIAB to send CC events for volume when you play a song.

Jim