HT Bypass

Ok here goes, my first ever post on a forum. I’ll apologize now if this issue has already been vetted or I just sound like the dork newbie I suspect I am.
I’ve recently upgraded my 2-channel system and want to integrate the HT system to decrease the number of speakers in the rig. I’ve been running two separate systems for movies/TV and dedicated music listening. I replaced a denon AVR 2311 CL with an Arcam AVR550 HT receiver because of the pre out section. I then ran RCA from the FR,FL and sub to the HT bypass inputs of a parasound P6, which then uses balanced XLR interconnects to a parasound A21+ amp. The left and right surrounds plus center are hard wired to the Arcam binding posts. Also connected to the Arcam is an Xfinity cable set top box via HDMI. The TV is a 10- year old Sharp Aquos connected to the Arcam via HDMI. I have not yet run the Dirac speaker balancing function.

The problem is I can’t seem to
eliminate what I think is a ground loop producing a constant, loud and unacceptable hum. The hum exists even when the power is off, and disappears if I forget the preamp and just hard wire the sub and FR/FL to the binding posts of the Arcam AVR. The hum also clearly disappears when disconnecting the Xfinity box, but not while connecting/disconnecting other inputs to the Arcam, changing out RCA cables, or changing power outlets. All components get power through a dedicated 20-amp outlet and a Monster HTPS 7000 power conditioner.
Arcam gratuitously provided a lengthy phone support session, but at the end said I can try updated HDMI cable (mine are quite old), or just settle for two separate systems- HT and 2-channel. The 2-channel works great, by the way.
I again apologize if it’s repetitive and open to links if anyone knows where I might read more.I hope that is a clear question and doesn’t leave out any critical information.