Thanks Steve. This is very helpful. In the case of the FC7 and the Akai, I’m 90% sure it was the impedance, rather than the polarity. I opened one of the pedals, got out the voltage meter, etc. As I recall, the FC7 doesn’t move the pot all the way from end to end, and with the 50K pot, the voltage didn’t go far enough to make it work. I tried some parallel resistors, but never found a recipe that worked.
Anyway, the KeyLab MkII was happy - and so am I.
FWIW, I bought a Nektar expression pedal to go with the Nektar Pacer controller specifically because I wanted to ensure success. Once I receive delivery, I’ll have three working pedals. Crazy! 