I have used unmodified Genesis pads without problems. B is the normal button and C works on some Amiga games. A can be made to work by swapping pins/lines 5 & 7...
http://psygnosisamiga.free.fr/games/hiredguns/instructions2.pdfThe ADoom docs also suggest putting a 470 ohm resistor in...
"A Sega Genesis controller may be use on the Amiga as long
as you swap lines 5 & 7, and put a 470 ohm resistor
between lines 5 & 7."
This allows a 6 button Genesis control pad to function on the Amiga.
