The poor balance of each instrument volume exists in some of the presets, when you are using ACMP feature. I believe that is what the software engineers, desired. It is not uncommon to have some instruments fairly soft in a mix, for the sake of clarity. However the balance of volumes in many of the styles doesn't feel right. In general, many of the volumes are just too soft. Out of 1 - 10, many styles have the loudest volume about 7 or 8, and softest down at 2. I don't really get why they designed the style volumes this way. Jumping styles in a song, will often make some instruments, drop so radically, it get's lost.
I too use the Genos in a different manner, (but do use ACMP).. I adjust the volume of each instrument to my liking when recording. I adjust the volume for maximum (for best signal to noise ration), and fix the mix in my DAW
However if you were playing Genos live, switching from style to style playing live, Irene's idea would be the solution.. Sometimes, I change the style, so that the bass uses the same instrument, for continuity. For many playing live, jumping styles, this probably won't make much difference, especially if your singing.. It will pass by unnoticed, because there are so many things happening anyways. It's only after repeated listenings, the ear will pick up on it.
I however, would not have the patience to go thru each style, re-adjust and save..