I don`t really need to transpose that much
But say i transpose +12 on my KB, it will play +12 on the softsynth. It doesn`t change the octave on the softsynth itself, just the keys pressed like 12 octaves higher.
I control my softsynths with my KB, and theres no problem controlling other synths either, it`s just about finding the right knobs with the right CV`s.
Softsynths with a Learn-by-midi-button is a no brainer though.