Noise Between Receiver/Preamp and Integrated/Power

I recently picked up a used Cambridge Audio Azur 551R (receiver) and a Cambridge Audio Azur 840A v2 (integrated amplifier). My goal is to use the 551R as my home theater receiver with the 840A as the power amplifier as it has the capability to set the inputs to be fixed (i.e. home theater bypass). The problem is that I am getting quite a bit of noise through the speakers with this setup.

The 551R and 840A are both silent when being used independently. I currently have an Airport Express and DAC as the source for the 840A and an Apple TV as the source for the 551R because it allows me to feed both from my computer simultaneously.

When I’ve adjusting the fixed input level on the 840A the noise seems to mostly be eliminated around -20 dB but the end result is having to drive the volume level on the 551R significantly higher than if it was on it’s own which doesn’t make sense.

They are connected with a fairly long set of cables that I plan to replace with a much shorter set to minimize any potential interference from the next of cables (that include power cables) that is the reality of my setup. I’ll also try plugging them in differently to see if that makes any difference.

Any thoughts?

Update: I switched in a different different BJC LC-1 cable that is about half the length and swapped one of the power plugs from the power conditioner to a power strip to give them different equipment paths to the wall plug. I didn't notice any change.