That was a bit of a throwback, but HDMI is much more preferable. Initially, you may think to use the headphone-out of the computer and I made the same mistake, but the Model 976's analog input section is pure analog. This means the EQ does not, and cannot, affect the analog inputs because they stay analog through and through and are not converted.

You can use the headphone-out to the 7.1 analog inputs to analyze two channels at a time and the frequency response of your speakers in your room. You can then make your changes in the Model 976's EQ, BUT, you would not be able to monitor, visually or audibly, your changes because, again, the analog inputs are not impacted by the EQ.

Ultimately, an HDMI connection is necessary to monitor and correct any changes made in the EQ. If you don't want to tinker to that level, and visually see the affect of your EQ changes, analog connections from a headphone output should work just to implement the changes recommended by REW. These changes will then audibly impact all digital audio inputs.