Why is it not possible to assign to each drum note, optionally, a different midi channel? So the question do the midi channels have to be track based, in a fixed way, or allowing per drum note midi channel assignments.

By default every drum note would get the track midi channel, as it is now. But then, the user should be able to set a difference, per drum note. diff = -1 would decrease its midi channel by one. Or using absolute values 1..16.

This would give the drum tracks a little more freedom. So you could still use certain notes as bass notes if you want, going to another synth, even sitting on the same drum track. This would be a bit like having more than only 4 tracks.