The Audire amplifier and preamplifier are both old enough to have just a two prong electrical plug.
Do you have a multimeter?
I assume the AC power plugs on the preamp and power are not polarized? They will plug into the receptacle outlet in either direction.
If you have a multimeter you should measure the voltage from the metal chassis of each piece of equipment to the ground contact of the wall outlet.
** Interconnects must be disconnected/removed from the equipment.
With the equipment powered up measure the AC voltage from the ground contact on the wall outlet to the metal chassis. Make note of the AC voltage reading. Turn off the equipment wait a few minutes then unplug the piece of equipment, being tested, and rotate the plug 180 and plug it back in. Turn it back on and measure the voltage from the ground to the chassis again. The lowest voltage reading of the two readings is the correct plug AC polarity orientation for the primary winding of the power transformer of the piece of equipment tested. Mark the plug for identification. Test both pieces of equipment.
Reconnect the interconnects from the preamp to power amp. Plug in the preamp and power amp plugs with the correct identified plug orientation. Check for hum.