Thanks for the responses everyone.
lovinthehd - I only have two sources plugged into the amp, the Marantz and the Opp. Both create a hum but the Marantz is definitely louder. Eventually I will put the Opp through a DAC with balanced output so I'm less concerned about the Oppo. The Marantz is already a two-prong plug and adding the cheater two-prong to the Willsenton doesn't remove the hum. Checking out the article tonight, thanks for the link.
DigitalDawn - I tried unplugging the cable and the hum is still there. Also, I do not have a cable box near this equipment, just streaming.
OHMisback - I'll try the cable positions. Not sure what you are offering in terms of a DIY build but I'l look at anything you send me.
highfigh/TLS Guy/PENG - thanks for the education, I'm learning a lot through this thread. Yeah, the Marantz has no balanced connection options. Also, the Willsenton only has one balanced input (I should have paid more attention to that when I bought it). Anyway, I'll probably pick up a RCA/balanced cable like you suggest PENG just to see what it does.
One other thing I think I am going to try is to attach a ground wire to the Marantz "earth terminal". There isn't a water pipe anywhere close (plus it's a finished basement and I don't want to tear into drywall. But there is a window within about 4' of the Marantz and I could run a ground wire outside that window to a copper rod I could sink in the window well area.