Just upgraded my Integrated Amp - now here a HUM out of my LUMIN - HELP

I run two systems in my home - System one is in my family room - my old Krell 400 xi died last month. I replaced it with the new Krell Integrated amp the K300i - Everything is better - soundstage is larger - highs are clearer with no harshness, bass is amazing, and midrange vocals are so real Carly Simon vocals bring me to tears! My problem is I now have a very low level pulsating hum between tracks that I cannot eliminated. I have isolated this hum to my LUMIN when i input its feed into the Krell. The low level hum follows the RCA jacks as I move them from inputs S1 to S2 to S3 on the Krell. My DVD / CD player on this system does not have this issue - it is stone silent between tracks. The only ‘odd thing’ about my LUMIN RCA inputs is they are long - 15 feet. I have my LUMIN upstairs feeding my second system via the balanced outputs. This system runs with my older Krell S550i integrated amp. The sound is stone quiet between tracks - no HUM.

i am running the Krell K300i at a volume output of 65 ( volume runs from 0 to 100). I then control my actual volume via my I pad mini via the LUMIN app. My sound is amazing , but this HUM just bothers me. Do I need to ground the LUMIN D1? Both systems get their power via two separate FURMAN ELITE 15 Power Conditioners. I am out of options. Any help would be appreciated