See if you can find a copy of Snooper. It's old but it's an amazing diagnostics utility that could also boot standalone from a floppy. It had a number of SCSI tests including device detection, controller testing and destructive disk testing (doesn't need a valid apple driver or filesystem to do this and IF it does anything then it means that the controller is fine and it can write and read back data on the bus)

Here's another possible twist that I haven't explored yet but might be worth mentioning: The Quadra came with a National Instruments NB-GPIB-P Nubus IEEE 488 controller card installed. I had removed that and set it aside prior to recapping the mobo and then forgot all about it until just now. Is it possible that because the system was previously configured with this bus card, the SCSI busses now cannot function without it installed? I have to dig it out from storage to reinstall it, but if it is extremely unlikely this is reason then I won't bother.

Click to expand...

Those are fancy cards if you have any hardware that supports GPIB. National Instruments still has the drivers and software available on their website but no, weather or not the card is installed would not affect the machine.