I had a similar setup as you, Aurender N100H feeding a Sboosted Qutest via Curious and other USB cables. For the most part it sounded great, albeit a bit harsh on some recordings. I'm not bashing USB but I moved on from it due mostly to the Aurender losing connection to dacs (several popular models) mostly when switching tracks quickly. I'd literally have to reboot the dacs. Probably an Aurender issue for sure.
I've read about Robb Watts impressive spdif implementation, as well as many opinions on other forums that the Qutest's least favored interface is usb.
I also looked at the Audiowise SRC-DX but ended up with an Aurender UT100 usb/toslink converter. I recently acquired their UC100 usb/coax converter as well. Both units solve all the connection issues and as for sound, with usb I only used the two warm filters, without usb all I use is the incisive filters. So I think there may be less "digital glare" with spdif.
Good luck with whatever you decide! That M3 does look awful nice :)