The A4000T have a bog-under-powered AT PSU. 8V in the +12V line? That doesn't sound right.
You'll expect problems in the SCSI chain with those low voltage rates (the terminators don't react the way they need).
Strip the PSU out, put some electric drain device in the +5V line (a car 50W lamp, for example) and turn it on. Read the voltages again to be sure. IF no right, swap it for a better power supply.
After that, be absolutely sure that nothing is in short-circuit when attaching everything back (one board at a time). Notice the A4000T (as any A3000) don't boot without a terminator on the SCSI cable and the straps in the right position for your setup.
