Hum/Buz fro JC1

Do nothing with the cable jack. You may still want to try disconnecting the coax cable at its first entry. If you have a ground loop that causes this humming, you will know it when you take out that cable. Your amp may not be sensitive to this ground loop, but your preamp may be, since you say the noise is gone when you disconnect the preamp from amp. Try it, if it's not a big deal. Other than that, it could be your preamp that's causing this problem. Tubes, other things......who knows. Also try a cheater plug as recommended above.

cheater plugs are just plug ends with the ground removed. you can buy them in hardware stores. It allows your 3 prong male plug to plug into a female end with two male prongs on the other end

Downunder, i did the same thing with 3 prong female by disconnect the ground wire so which means that there is no ground feeding the amp.

it could also be that your rca jacks from preamp to amp are not seated all the way in the amplifier input jacks. If you are using locking type rca connectors, then you may want to make sure they're pushed all the way in. Some input jacks make it seem like they're in all the way, but in reality there's still room.
Also, try connecting your cd player to the amp(but don't hit play) and see if that causes the same buzz as the preamp does. If not, your preamp is causing your problems.

The best info I have seen about cheater plugs is not to use them. The shock from an ungrounded chassis can be lethal. If the interconnects are well made and properly seated, the next step would be to use a good quality power strip and plug all the units into it. Generally that should take care of the problem unless it is a real design flaw in one of the system items. Good luck.