The Focusrite 6i6 is probably the least expensive option, which also has a S/PDIF coaxial connection. There's a few good reasons to use S/PDIF, or to at least have the option.
Your KPA and Mac connected to the Focusrite, and through the HS8's, would sound great. The Focusrite headphone output is not the worst, but it's not great. You really notice the difference when you plug into the KPA headphone jack directly, in an A/B scenario. I'm fine with the Focusrite headphone jack, it can sound good with adjustment.
You could also plug your headphones into the KPA directly, and use the AUX in on the KPA, and then route your Mac into the KPA. No recording option that way, but it would allow you to play with backing tracks, or along with music.