Just in case this helps anyone... I've just spent a week screwing with sense pins, SwitchRes, Multiple Scan Software disks and differing versions of Mac OS to work out why my Quadra 950 would only ever detect my Apple Multiple Scan 720 as an RGB 12" and therefore only allow email@example.com.
It turns out that you need to disable DDC in the monitor itself. When powered on, go to the OSD and scroll to the Information tab. Scroll to DDC and then make sure it is OFF.
Cold boot the machine and you'll have all the resolutions your Macintosh is capable of.
It seems that DDC interferes with how the Quadra manages to determine what resolutions the monitor should display.