As long as your PC software is setup correctly to pass the PCM unmolested to the USB port, it really doesn't matter what PC you are using. Most, if not all, popular software players can be setup correctly. All you are doing is ripping CDs to .WAV or some a lossless compressed format (eg. FLAC) to disk and when playing all that happens is sending the resulting PCM to the USB port and onto the DAC. Popular free software players are JRiver Media player an Foobar 2000. I'd stay away from iTunes because it's a "pig" and overkill for just sending PCM to a DAC. However, I guess there's no real harm in using it if you don't mind being forced into using Apple's formats.
I have a QB-9 and love it. Didn't know anything about the W4S DAC2 till now. I see that it has an asynchronous USB input also, plus many others. Cheapest asynch USB DAC that I've seen (haven't been looking). With Ayre products you get great sound with an upgrade policy and customer service that are second to none.
One red flag for me for the DAC2 is "proprietary drivers" for the various O/S's. Looks like they didn't license the Wavelength async USB software for inside the DAC. So it's definitely an unknown quantity. And it's kind of a swiss army knife DAC (many inputs, volume control, pre-amp) compared to a dedicated USB DAC like the Ayre.