I had a problem with my GVP HD8+ PSU. The solder joints on the female IEC plug became loose. I had to re-solder them.
I would still check the power supply and resoldering IEC plug is about the easily thing in electronics to solder (the solder joints are very large). I have seen a couple of these power supplies have loose connection. This loose connection can often cause strange effects.
Only one of the voltages maybe a problem or a ground or even the board may not be getting enough current.
I would get anything from the hard drive not spinning (controller and ram would check out just fine. Even the hard drive would identify itself with SCSI Mounter) to a variety of Guru errors.
Remember the A500 HD 8+ and A500 should not co-mingle their power supply, only signals.
improper power to the A5000 HD 8+ will not necessary keep the A500 from coming on