Two different conversations entirely to be had: USB or SPDIF?
If SPDIF or AES/EBU, the Aurender is up there with the best of the best. Having said that, some of the top Esoteric transports from the heyday of CD - and some of the clock-upgraded TEAC models around - are very clean-sounding. You can attach one of these to any DAC and they will make a bigger difference than doubling or trebling the price of the converter. You can upgrade a Sonos with a similar clock and power supply and turn it into a very fine source.
If USB, we're largely asking: 'how good is the DAC's USB implementation?’ Which is why there's no level playing field on which to make a comparison. And unless you're very careful to isolate the DAC from any time-domain or power influence, the USB transport (computer) will quickly make its presence felt. Anyone who's heard a properly optimised computer will tell you the computer is part of the equation with almost all DACs. If you want to polish the last 10-20% of grunge out the system, it's essential not to skimp on the PC.
Mac or PC isn't the issue: the big question is how is it powered. It's not possible to power a motherboard cleanly with a single-input supply: if you see a 'music server' with only a single-rail input, don't be duped: it's half-specified. Therefore, Windows or Linux on an ATX-powered board with a three-rail linear is (unfortunately) preferable to any species of Apple.
If USB isn't your DAC's forté, combining a tidy computer with an SPDIF converter from the top drawer (Audiophilleo, SoTM, Sonicweld, Empirical) is surprisingly comparable to the better SPDIF transports mentioned above - or the dedicated streamers from Naim, Linn, Weiss, etc. Just much cheaper.