In my experience, the rotational adjustment causes a minimal change in sound. I've tried the FC with and without preamp (Merchant Vertical Bass, Oblis) into a GK MB 150 with no difference in sound as far as I and the others in the band could tell.
I've also run the
VB into the AI Clarus SLR/WZ 110 with the same results re the pickup rotation. No preamp needed w/ AI. With my LaScala (EPs) at home, I could tell a tiny difference upon pickup rotation, but not enough to bother with. Same w/ preamp test: no discernible difference.
The FC is very amp friendly; plug in and play.