I will get around to Octals, and hopefully this year. Kudos to you!
Since the C/C2 is common grounded, the Cornet is probably seeing a loop from the ground lines of the ICs to their source. Does it hum equally loud with each IC?
On my tonearm IC's I have the line and the neutral on individual wires and the shield ties to only to the preamp's star ground and not to the right or left RCA grounds. More importantly, the shield is floating at the tonearm.
I suspect one of three things. Each of these are fairly simple to check.
First off, if your ICs are shielded and the shielding is connected at both ends of the wire. No matter what you do you will end up with hum. If so you simply need to detach the shielding from one side or the other of the wires. Also make sure that both shielded ends go to a short clean pathway to ground.
Second, did you try reversing the wires at the cartridge? By that I mean plug green into the red and red to green. Do this on both channels and see what happens to the hum? I have seen some tonearms accidentally mis-wired so they will hum until the signal is properly oriented. You can actually check this with a meter. Pull the red cartridge clip. Does the right RCA center tone to the red clip? Does the right or left RCA center get neutral or shield? These tests will let you know if the arm is wired correctly.
Third make sure you use a ground star approach to the turntable. The motor ground should go to the main bearing and the tonearm. All three should be interlinked. Now the table needs to be on the same ground plane as the Cornet, the preamp and your amp. With things star grounded and on the same ground line, now you can start pulling grounds off the star points until you find the culpret(s).
I hope this helps.
