There are products that are meant to clean up the usb signal between a PC and the DAC. They basically clean up noise and timing issues with the PC usb interface. If the interface is very good, then the device will have little or no effect. If the interface is not very good, the device can clean up the signal. It can also re-clock the usb signal, which should not be a problem if the DAC has a good usb interface with good timing circuitry. Some people report no difference when using these devices and some people report a significant increase in sound quality, including "open and airy" as you note. Some also lift the ground from the computer, since that can, in some cases, have significant noise on it which can have an effect on some DACs.
Again, these devices are meant to correct deficiencies in usb unterfaces, which may or may not be present in your system.
One such device is the ifi iPurifier3. If you read the reviews on Amazon, you will find people who heard no difference and you will find people who say it made a big difference. As usual, it depends on your system and your hearing.