Hum in left channel with source turned off

I just hooked up a REL sub to my Sonus Faber Musica integrated using the high level cable connected to the speaker terminals. The gnd of the REL cable is connected to the right speaker gnd. I noticed that, if the Musica is turned on when the DirectStream DAC is turned off, I get a noticeable hum in the left channel. If I disconnect the REL cable, the hum goes away. When I turn on the DSD, the hum goes away. BTW The REL is on all the time. I think I remember that it is OK to not use the gnd wire for the REL connection if necessary, but it’s a pain to get behind my equipment, so I am avoiding it. I am assuming that the REL connection is creating a ground loop & wonder if I should worry about it. After all the hum goes away when the source is on. It just bothers me & I am trying to understand why having the source off or on makes a difference. I am an electronics tech, so I should be able to understand a technical explanation. Not having schematics for the REL input stage or the DSD output stage doesn’t help. Any thoughts are appreciated.