One thing I've noticed -
Let's say R1 Brass on Registration 2 / Volume set to 60, because I don't want the brass blaring ....
Let's say R1 Piano on Registration 1 / Volume set to 100, because it just needs that extra little punch ...
So - I'm playing Reg 1, piano, and I have to transition to Reg 2, to play the brass ... and at the end of my phrase, I'm sustaining with the piano .... and I punch Reg 2... The brass comes blaring in... UNTIL I release and continue to play... OR vice versa ...
What I've found I am needing to do is to make sure the volumes of everything from registration to registration is fairly consistent, otherwise, I can expect strange results when moving from one registration to another...
In your situation - unless you "normalize", you'll always get different results ... No ONE audio MP3 is recorded the same... In my opinion, your "ear" is probably the best tool... BUT, you take it a step further, and balance all of the instruments across ALL of the registrations you use for that MP3 ... AND of course, this puts you in the neighborhood ... and from there, use "live control" to manually adjust on the fly...
I LIKE how you are doing this though ... it's a great idea for what I want to do.