I have an external keyboard which I play. Midi Out of BIAB goes to the keyboard which is used for Acoustic Bass and Nylon Guitar. The output from both is controlled by the KB master volume. However, I am able to adjust my keyboard playing with a separate volume slider on the KB. I don't know the Roland but am sure it should have something similar.

Within BIAB, there are the the volume spinners which have been discussed. There is also a Midi normalize feature which may help. It is found in Opt|Preference|Arrange menu. Check the box and play with different numbers. I found 75 to 80 works for me.

R

p.s. The Combo control should not affect your solo sound unless it is being looped back through BIAB. What are your Opt|Midi settings?