I use my digital keyboard in MIDI mode (With Garageband, on a Mac, similar are available on PCs), and use whatever sound device is plugged into the computer. Speakers, headphones, etc. For me this comes out better quality than the headphone socket on the keyboard, as my keyboard is a weighted key, stage piano, and quite old. This approach would work well with your USB headphones. Any reasonably modern quality digital piano keyboard should work like this.