I never had a problem "booting" my SE/30 with a IIgs ADB keyboard attached, but sometimes the keyboard would work and other times not. I know you're not supposed to unplug and replug an ADB keyboard while the Mac is powered up, but when I would do that, the keyboard would start working again. I ultimately, took the time to open the keyboard and examine the circuit board. Sure enough the electrolytic capacitors had leaked. I replaced them with tantalum versions and tested. The problem was gone, and so far it has not come back. This may not be the source of your problem, but it would not hurt to replace any fluid-filled capacitors with tantalum equivalents.