Hum from phono amp with Stellar M700

Hi PS Audio team and forumers, firstly a happy new year 2019 greetings to everyone.

I’ve recently set-up my Rega Planar 3 with Lounge Audio LCR3 + Copla MC step-up phono preamp into a Primaluna preamp and into the M700 monos. The M700 is hooked up to a single REL subwoofer without the ground wire connected.

I’ve noticed a small hum whenever my preamp input is selected to phono input. No hum at all when it’s on CD input. The hum is there regardless the turntable motor is on or off.

My phono preamp is not connected to any ground eventhough there is a ground connector. I recalled reading Class D amps are not reference to ground. Could this be a ground loop issue? How can I solve this? Can I connected the ground connection of the phono preamp to the M700 chasis or it’s -ve output terminal? I don’t seem to notice any hum when I was using a Class A power amp previously.