hmmm, ok
are you using the scsi cable that came with the machine? might be worth getting hold of another one. mine was all folded up when i got it and might have had a couple of internal lines broken... seemed to work when straightened, but other than that, it didn't... anyway, i replaced that with another cable, and took the headers off the cable to use on another new cable for all my devices.
and one of the things i remember was that swtiching the dip switches to "on" switched off the functions. most irritating. one other thing to consider, (and boosted my scsi speed) was switching off the termination, and putting an [active] scsi terminator on the scsi port on the back. other than that, its just make sure that none of your devices have termination enabled apart from the last one(s) on the chain, but i guess i'm pointing out the obvious there

(scsi controller counts as a device)
oh and if anyone does seriously want a Scsi CDrom or DDS2 tape drive, send me an email, and if you're in the UK i'll ship it free, outside the UK and i guess the postage'll cost more than a new drive! two of each, and they're just sitting on a shelf not doing anything...
oh crap, just thought. it might be worth one of your devices suppling "term power" to the bus (should be a jumper for it on one of your drives) this would enable the proper termination if you are using an active terminator.