Rex, On your question #1 please bear in mind that most Japanese products come with three prong plugs but few have the ground attached to anything. Many don't even have a third, ground connector at the unit.
Artar,
I recommend the following procedure to at least identify the culprit.
1. disconnect everything except the amp to speaker wires. Put shorting plugs into your main amp and turn it on. If you have no hum, the culprit is not the amp.
2. Connect the preamp to the amp after removing the shorting plugs. Turn the preamp on and then the amp. If you have no hum, neither are culprits.
3. Plug in one front end device. If you have no hum, with it active, it is not the culprit.
4. When you add something that causes hum, it is the culprit. It is possible that it is that unit's interconnects, so try others. Also it may be the only grounded front end device. Try a cheater plug. If the hum does not go away, try reversing the cheater. If none of this works, you have a problem with that device.
If the hum goes away with the cheater, which will typically be the case, you have an issue--do you need it grounded? Rex would answer one way and I another.
You might also find that grounding the unit to the amp may cause the hum to go away. Also a resister on the ground may solve the problem.
Artar,
I recommend the following procedure to at least identify the culprit.
1. disconnect everything except the amp to speaker wires. Put shorting plugs into your main amp and turn it on. If you have no hum, the culprit is not the amp.
2. Connect the preamp to the amp after removing the shorting plugs. Turn the preamp on and then the amp. If you have no hum, neither are culprits.
3. Plug in one front end device. If you have no hum, with it active, it is not the culprit.
4. When you add something that causes hum, it is the culprit. It is possible that it is that unit's interconnects, so try others. Also it may be the only grounded front end device. Try a cheater plug. If the hum does not go away, try reversing the cheater. If none of this works, you have a problem with that device.
If the hum goes away with the cheater, which will typically be the case, you have an issue--do you need it grounded? Rex would answer one way and I another.
You might also find that grounding the unit to the amp may cause the hum to go away. Also a resister on the ground may solve the problem.