hum from input echo

Hi all,
 
I'm just doing a bit of tracking in X3 this evening.   And I have my Roland XP synth plugged right into my Tascam US600.  When I arm a new track and turn on the Input Echo,  I'm getting this weird little 60-cycle type hum.  I've turned off every other electrical item in my basement studio, monitor, keyboard, fan, dehumidifer, etc.  and it's still there.
 
Granted, I can turn off the Input Echo and track just fine, but then I can't hear and monitor through my interface.  Am I missing a setting or troubleshooting step here?

Classic ground-loop symptom
 
Do you have all gear running from a single outlet?
That's the easiest way to avoid grounding issues.

If there's a ground-loop, an unbalanced cable connecting the keyboard/audio-interface will reveal it.  
If you have a DI box with a ground-lift switch, you can use that to test.
Plug the keyboard into the DI box... and plug the DI box into the audio-interface.
If the hum disappears when using the ground-lift switch, you've got your culprit.
 
If the issue can't be solved by moving electrical cords to the same outlet, use a pair of DI boxes (as above) to solve the problem.

I do have an slightly-old SignalFlex passive DI box that I tried - doesn't have a ground lift switch on it.  But the hum has went away on the tracks.  Is that a decent enough workaround?   Seems much improved to my ears.