That I know for ordinary 1 button operation, and my Comp Pros are wired exactly that way; but according to
http://wiki.icomp.de/wiki/DB9-Joystick, two button joysticks for C64 and Amiga
should connect POT X to VCC when the button closes the circuit, not close it to GND as the Sega gamepads do.
Is the wiki-page wrong, or the Amiga tolerant? (I assume the latter.)