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.
tclarkmd
I had a similar issue, only the hum was completely isolated to my xFinity system. I was in the process of installing a dedicated circuit to power the entire system when the xFinity techs came out for an unrelated issue and ended up running a new line from the street, no charge.
They told me their previous system was improperly/poorly grounded and now my system is dead quiet.
I am still running a dedicated line, however.
I would do my best to follow the xFinity from the street to the box and be sure the installation is clean, isolated, and properly grounded.
Next, I would install and use a dedicated 20amp circuit for the system.
Last resort, I believe there are plug-in units that are meant to address ground-loop issues. I have never used one.
Get a cable line ground isolator and say goodbye to the hum, you can get this from Amazon for $15 . 
Thank me later!

TII 220 Ground Loop Isolator for Cable TV 
I second jdub39's response!  I chased a ground loop hum for months, trying all different kinds of configurations and fixed it with a cable TV ground loop isolator as well.
BTW- thank you all for your help and suggestions. Tried to send the signal from Xfinity box to AVR via optical, which eliminates the hum, but no video. Looks like the ground loop isolator is my next move. 
Jdub39, I’m here to thank you later. The $15 isolator solved the issue. Builds my faith in community.