I maybe wrong, but isn't the DAC in the headphones? The source will just pass the digital signal over USB. I've read reviews saying there is no difference between Bluetooth and USB, but a bitrate over USB should make a difference.
Indeed there is of course DAC is in the headphones.
I think he's saying if you go wired you could use an external DAC of a higher quality.
I've seen a review stating wired over usb is a noticeable step up in quality over wireless on these headphones. Looking forward to testing that claim.