I'm about 10 years late to the party, but just discovered a flextone III 2x12. Sounds amazing.
Anyways, I bought an fbv express mkII to flip between presets, but the amp would not accept it. It would power up, lights on etc. I calibrated the pedal fine. But it would only read which channel was selected. It would not actually change any channels. The expression pedal also did nothing.
I've since sold the pedal and the guy said it worked fine for his amp. I'd like to be able to switch channels with my feet as the amp doesn't seem to like my drive pedals.
Any ideas on this?