To have 3 toms, I can indeed load 2 toms from the Vintage kit to the Southern 70’s kit (not the other way around as the Vintage kit only has 2 tom slots).
However, the mics and room sound is quite different from one expansion to the other. Loading in kit pieces from different EZdrummer expansions I find, doesn’t sound as good as using separate instances, to get the room sound where it was originally recorded.
