Rev. 8A should work without any tweaks with the busboard, it's the only rev. that supplies the 7 MHz clock to the expansion port, which may be necessary for some CPU cards; no idea if Blizzard needs it. Functionally my A500 and your A500+ should be similar after the tweaks I've done (!?).
The disable key is 0 (zero) - I mixed it with A1200 Blizzards

Are you sure to have the Blizzard librarys installed - the only idea that popped into me.
ps. - Blizzard seems to have clock jumpers cclock and eclock, open by default. Perhaps they could be closed for test. Maprom and SCSI jumpers should not affect to the booot process, if you don't have other expansions in the tower.