PowerMacG4 won't start without old Cinema Display attached

PowerMacG4 867dp (Mirror Door)

NVidia GeForce4mx w/ADC+DVI

20" Cinema Display w/ power switch, on ADC port

Acer 23" G23HL, on DVI port

I replaced my failing ADC with a new display (with much-appreciated help from this board!) Works fine as dominant display (currently not mirrored) but I need Cinema Display attached and powered up to run system. The Power Switch on tower lights, but won't shut down system, or start it without ADC attached. (It won't control system at all, only Power Switch on ADC works) Nothing in Preferences seems to apply. Should I just duct-tape new Display over old ADC and move on??? THANKS!