I picked up a Bricasti M5 two and a half weeks ago. I am using a Chord Qutest DAC being fed by Transparent Coax cable. I have not tried going the USB route. In my research I ran across the following which you may already be aware of:
(1) Christiaan Punter’s in depth review of the Bricasti M5 back in 2017 at his Hifi-advice.com website. He gives the M5 a rave review and states his preference for USB.
(2) Brian Zolner, principal at Bricasti, has stated that with respect to the M5, AES/EBU is the best way to go, with spdif being a close second.