Q: 15 inch ADB studio display issue with Power Mac 6500

By "Apple Desktop Bus Version," do you mean the display that also had an ADB port for connection of the keyboard/mouse and the ADB pigtail cable split-off at the end of the cable?  I'm assuming that it's connected to the onboard DB-15 monitor port and not to a PCI graphics card.  Do you have the checkbox selected to show unsupported resolutions?  Perhaps the Monitors Control Panel has become corrupted and/or its Preference file.  The easiest thing to try would be deleting the Preference file and restarting, so that a new file is created.  If that doesn't resolve the problem, boot from the installer disk and delete the Monitors C. P. and reinstall from the disk.  The software utility "TomeViewer" is useful for extracting individual files from the disk's installer tome(s), when only needing specific files.  The other thing to remember about your 6500 is its internal 4.5-v alkaline battery on the motherboard (Rayovac 870/871).  Eventually, the battery discharges and unlike the Apple computers that had the 3.6-v, ½AA lithium battery that had longer service life, that alkaline battery will eventually start leaking when its charge is depleted.  With the vertically-mounted motherboard in the 6400/6500s, the solution will drip straight down and its corrosive effect will damage the components in its path.  If you haven't replaced it in a few years, I'd do so to prevent a bigger problem.  Since you installed the Sonnet upgrade card, you're aware of the battery's location and velcro mounting base.