Turntable/Subwoofer Ground loop

I have a Rel B1 that is hooked up to Cary MB500 monoblocks via the high level connections.

I am starting to get a ground loop hum from the sub when I play a record. Only when the needle hits the record does it start. I have re-checked all connections, what could it be?

All the ground wires are connected what I think is correctly, but the high level connection from the amps to the sub has the wires stretched quit a bit to reach the amps and the ground.

Should I ground the Sub connection to somthing different, like the rack???
I had this same issue with my Rel Stadium III, after I changed from at stereo amp to mono blocks. Take the ground wire and connect it to the grounding terminal on your preamp. I also hear that Rel makes a cable for it with 2 ground wires but I have never seen it.
Is the hum the typical 60Hz hum? If it is 60 Hz then it is a ground loop issue. Otherwise, you might try improving the mechanical isolation of your turntable. Low frequency feedback from the sub could be exciting your turntable. Another quick check- invert the phase on the sub and see if the hum stops. That might also indicate a mechanical feedback issue.

I think you may be right. I isolated the phono cable and most of the hum went away, I need to try inverting the phase of the turntable. Good advice, thanks.
I think you want to invert the phase of the sub, instead of turntable.
That's correct. just a quick way to see if the feedback is mechanical. most subs have a phase switch.
Sorry, I meant sub, I did that, but there is still some hum.
Humming more again after some improvent. Only from the high level input. When that is disconnected, no hum, and no hum from any other componenet, or when playing anything except the turntable???