I noticed the same problem and shutting off the backlights stopped it in it's tracks. It's not a classic ground loop (wrong frequency), and the fact that it gets stronger as you bring your foot closer to the softstep (no need to touch it) would indicate so as well. Plugging the interface and the softstep into the same - externally powered and grounded - USB hub stops the buzzing for me it as long as I touch my guitar.
IMHO it's not a classic grounding issue, but rather electromagnetic interference from the LEDs (or whatever it is) that do the backlighting, like standing in front of an old CRT monitor. What we are hearing is the frequency of the light emitted by the backlight and as you bring your foot closer to the softstep, it's inducing electromagnetic interference on your body, creating a field difference between you and the guitar, making the pickups transmit the (180° phase shifted) buzz.
Now, with everything in the same powered hub, if I put my guitar far away, the hum is almost inaudible, but if I keep my foot on over the softstep and reach for my guitar, the closer I get, the louder the buzz gets, until I touch the guitar, at which point it goes away.
Putting my interface and the softstep on the same hub and powering that hub externally aolved it for me. I may have to add though that I'm using the hub on my old Dell monitor, which has a three-prong grounded power chord. I'm a little worried a non-grounded hub power supply may not have that effect? We'll see...