Minimizing noise with USB DAC?

I suppose this is more of a pro audio than consumer-grade audiophile question, but the tech is relevant regardless:

I'm getting a low, constant hum from my speakers. I hear it when I plug in the USB from my Focusrite Scarlett (DAC, essentially) to my laptop.

What I'm wondering is

1. are the more expensive audiophile-grade cables capable of reducing computer noise/interference

2. are there other ways of reducing noise?

So far I have tried switching ports -- the best option is this one, on the other side of the laptop from the power supply.

Being that the Scarlett is powered through an external supply, I wonder if there is a USB utility to stop power from flowing through the port but keep communication going?

Any ideas?