Mild hum/buzz preamp issue?

I just bought a MoFi StudioPhono to upgrade from my built-in receiver preamp and after setting it up I'm getting a low hum that increases/decreases with volume going up or down. Its not super bad but it is definitely enough for it to be annoying and for me to try troubleshooting it for several hours.

My setup is:

Project Debut Carbon DC w/ Ortophon 2M Blue
StudioPhono preamp
Onkyo TX-8511 reciever
Blue Jeans LC-1 Audio Cables

This is my first post so I'll try to keep the situation concise and list what I've done to troubleshoot.

I have it set up with a pair of 3 foot LC-1 RCAs going from turntable to StudioPhono then another pair of 3 foot LC-1 RCAs going from StudioPhono to the TX-8511 receiver non-phono input. The ground is going from the turntable to the StudioPhono, but I have also tried it from the turntable directly to the receiver. The ground cable is a standard 12-gauge stranded THHN copper wire (does length of the ground wire matter?). The hum obviously gets worse when I remove the grounding cables. If I disconnect the RCA cables from the output on the StudioPhono the hum stops. If I plug the RCAs back in to the output and then disconnect the RCAs from the input comping from the turntable the hum continues. I've tried flipping the AC plugs for the receiver and StudioPhono, plugging one into the wall socket and the other into a power strip, plugging them both in the power strip and again both into the same wall socket panel.

I'm assuming that it has something to do with the preamp since the sound continues when the RCA cables are disconnected from the preamps "in" and stops if they are disconnected from the "out".

Thoughts? Is it standard to get some electric sound when turning the volume up moderately loud?

I'm also curious to see if anyone has spent any decent time with this preamp and their thoughts...

Thanks for your help!