Hi Martin
I have a Qutest and have gone a little way down the rabbit hole on USB vs SPDIF. An important consideration for me is my preference for HQPlayer and upsampling to the maximum PCM rate of the Qutest 705k/768k. Using USB I was often the victim of brain frying white noise on occasion, normally when the base rate switched from 44.1k to 48k family.
I noticed some discussions about a device that converts USB to dual BNC (SPDIF) namely the SRC-DX. The thread below on AS is one such and folks on there way more knowledgable than me claim that using the dual BNC bypasses the Qutest’s Amanero USB chip, allows 705/768k and perhaps offers an improved sound.
I have the SRC-DX in my set up and yes I think it probably sounds a bit smoother (though not sure I would pass a blind test), but it definitely avoids the white noise issue on USB and illuminates the Qutest source light in a lovely turquoise (dual bnc).
Also interesting to note that the M-Scalar outputs using dual BNC rather than USB, presumably there is a reason for that?