Subwoofer dual wired hum question

Hello

I have an interesting dilemma. I have a Yaqin tube amp and it doesn't have subwoofer output, so I 'biwire' one et of speaker terminals on the Yaqin and just run speaker wire into the Sub. All good there. 

I only use the Yaqin for music. Everything else goes through a Vizio soundbar that has a RCA style sub out. When I run a single, shielded RCA cable from that to Sub, I get a hum.

Yaqin, Vizio, Cable box etc plugged into one output. Sub plugged into different outlet across the room. Cable box has a coaxial isolation transformer. 

I did some research - sounds like a common ground loop issue solvable with an isolation transformer. No problem. 

But before I order a blue jean iso transformer, I thought I'd check: is the fact that I am running speaker wire from one source and an RCA from another potentially causing the hum?

any help or thoughts appreciated.