Voodoo Lab has stated that you can use the XLink (2A) on their power supply to power the QC. They mentioned this in a comment on Instagram:
“X-LINK ports will deliver up to 2A in total between the two shared expansion ports. The actual current draw of the Quad Cortex is 1.8A. This was measured on our end and also confirmed by Neural. 3A is the rating of the wall wart that ships with the Quad, and that is the rating chosen as the published requirement. 3A is not the actual current draw of the Quad. The wall wart current rating is commonly used as the published rating for effects devices by manufacturers. There is no way to damage the Quad Cortex by powering it with X-LINK. The only thing to keep in mind when powering the Quad Cortex with X-LINK is that you should leave the second expansion port empty.”
Neural Dsp support, in an email I sent them, confirmed this information.