If you were running the laptop on AC a worthwhile experiment might be to try running it on its battery. I'm envisioning not only that the AC power supply might be introducing noise into the system, but when on battery power the computer might operate in a somewhat different manner internally, in the interests of conserving power, conceivably resulting in less digital noise and/or jitter on its USB output. And if the USB interface of the DAC in your C50 is not asynchronous, it might be particularly susceptible to that kind of issue.
Best regards,
-- Al
Best regards,
-- Al