No, I want to be able to listen to more than one input at once, which is why I wanted a mixer. I use Mouse Without Borders to control multiple PCs at once and I'd like to be able to hear multiple PCs at once too.I assume you wish to only listen to one at a time, then the Schiit Modi/Magni might be the thing for you! Magni being the headphone amp, and Modi being a selectable 3 input DAC. Connect USB to whichever computer is closest and let the Modi be the sound card, Optical to whichever device is next closest, and stereo RCA to the furthest. This is assuming the PCs have an optical port, which is now pretty standard on many motherboards. The PS4 does for sure.
About $200 shipped for the two units, and you'll find many folks here run that combo myself included.
I'm not really that interested in a USB DAC, that's a fallback option, but I'm actually pretty happy with the headphone amp on my Soundblaster Z.