Similarly, if there is hum with the SS on the laptop but no hum with the SS on your desktop, you can eliminate the common elements of that setup as the cause of the hum. It can't be the softstep that is at fault if it hums in one setup but not in the other. That's science. There's nothing to belive or not believe about it.
That's not to say that you may not be hearing hum from the softstep, but the fault will be elsewhere. Like I explained, if the power draw of the SS is higher than what your laptop is providing, you may hear hum from the SS, reflecting it's power draw, but not by fault of the SS. Like how the room lights flicker when you turn on the hairdryer: It's because of the hairdryer's power draw, but it's not the fault of the hairdryer that your appartment doesn't have enough current to absorb such an event.
And finally, whether the laptop is plugged in or not has no bearing on how much current the internal electronics can provide. A lot of USB-ports on laptops are underpowered by design. Try an externally powered hub. That's the only way you can eliminate this as the cause.
The bottom line is, if the SoftStep works perfectly on your desktop, the Softstep simply works perfectly and there's a problem with your laptop setup. Simple as that. No piece of gear can provide for shortcomings of every other piece of gear (or warn against them).
- Turn off the backlights to see if the poroblem persists. If it does, it's not what we're talking about here.
- try an externally powered USB hub and power your Audio interface externally to eliminate power issues on the USB hub as a cause. Make sure it's plugged in to the same sockets as all your other gear you're using. Try all USB sockets on your laptop.
- Try it without the audio interface plugged in. Does it still hum?
Then report back. Right now you are blaming KMI and speculating on causes against all indications, which are that the SoftStep works fine and your laptop has issues.