Ground Loop?

I had similar problem with a preamp. The solution was purchasing an isolation transformer. 

Perhaps run a temporary drop cord from another outlet to the phono preamp. Costs nothing and maybe the problem goes away? If the problem remains, it’s one more resolution to scratch off the list.

Ground problems are the worst.  Sometimes it can take a while to sort them out.

The original post states "Different TT but still has the issue whether it's connected or not."  So, this means that you get the noise with no turntable connected?  The way to test for sure is to short the phono input.  If the noise persists it's not from the TT, connecting cables, or TT ground.

Here are some other things to try to see if there is any change:

(1) Use the standard AC plug to ground the line preamp and then use "cheaters" to lift the ground for the phono stage and the power amp.

(2) Use a separate single wire to connect the ground lug from the phono stage to the ground lug of the line preamp.

(3) Use a separate single wire to connect the ground lug from the phono stage to the house AC ground (can use the center screw of AC socket plate).

(4) Use a separate single wire to connect the ground lug from the line preamp to the house AC ground.

Maybe you can find the magic combination.  Best of luck.

I once had a ground loop thet crept up when I had my preamplifier on one circuit and my amplifier on the other. These circuits came from different phases in my main breaker box. When I got the amp and preamp on the same phase the issue went away. Don't know if that helps your situation, but I noticed you had two different feeds for components so it may be worth looking at that. Try plugging all components into the same outlet and see what happens.