last_lemming,
I have another test for you if you have not already tried it.
With the preamp plugged into the AC outlet but powered off, not in standby if it has a standby feature, do you get hum/buzz through the amp/speakers?
I believe if the preamp has AC line filtering it is done near the IEC inlet connector ahead of the main power on/off switch. I am not 100% sure of that though.
Post back your results.
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Voltage test measurements.
With the amp and preamp powered up check for AC voltage at the wall receptacle.
For the test if both receptacles of the duplex have plugs inserted in them slightly pull the preamp plug out just a little so you can touch the meter test lead probes to the far outside sides of the hot and neutral blades and the equipment ground pin of the plug. Caution, be careful you are dealing with lethal voltage. **Also I assume the wall plate is plastic or nylon. IT Can Not be metal.
Tests
1) Set meter to AC auto voltage. If the meter does not have an auto voltage setting set the meter to the closest scale above 130Vac. Setting will probably will be 150Vac.
2) Next with both the preamp and amp turned on, powered up, measure the voltage from the hot to the neutral at the wall receptacle. Write down the measurement. This measure will give you the loaded idle voltage of the audio equipment at the duplex receptacle. It should be around 120V. Above 124Vac is not considered good.
3) Next measure the Voltage from the Hot, (smaller straight blade slot hole of the two slots of the wall receptacle), to the safety equipment ground. Write down the voltage measurement. It should match the voltage reading as you got above. Though depending on all the combined loads connected to the 15 amp branch circuit it could be slightly higher due to VD, Voltage Drop, on the neutral wire of the branch circuit.
(This measurement also tells you the receptacle AC polarity is correct.)
4) Next measure from the neutral, the bigger of the two slot holes on the receptacle, to the equipment ground. For this reading hopefully your meter has an auto AC voltage setting. If not start with the scale used in the two previous tests and reset the meter to the lowest appropriate lower AC Voltage scale. Hopefully working down to an AC millivolt scale. Write down the voltage. The closer to a zero reading the better. Though depending on the VD on the branch circuit, the higher the neutral to equipment ground AC voltage measurement will be. Hopefully it is not above 1volt - 1.8 volts.
Post back your voltage measurements.
PS, if you want true loaded voltage measurements play some dynamic music at a moderate to above moderate to loud listening level. Then repeats the above test measurements.
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