Yes Ack you are right.
The hum is in both speakers.
This item is also detected by the distributor but no solution till now.
I did all the things you said except adding a low impedance.
I think is the same you get with the Quantum QB4 (a low impedance resistor in the earth connection)
What is the value of the resistor you placed?
When i connect the pre to poweramp (and speakers) the humm is already there (volume set to "0").
I had a hum coming from one of my 360 mono blocks. It went to the importer (More Music) and they had to send a part to Spectral. I really don't know what was wrong with the unit. But one thing I do know: the hum in my speaker was caused by a defect amp.