Now, I do understand your application....and yes, that apparently is your only option, because the analog audio out jacks on the back of your unit don't provide a signal from digital inputs (I downloaded the manual.)
What I suggested was the typical set-up for someone using a CD player connected to the analog inputs of a receiver (typical for stereo music listening) and would bypass both the volume control on the receiver and the headamp and give better results.
What are you trying to gain, though, by setting it up this way versus simply using the internal headphone amp in the Yamaha and an extension cable? Just to provide local volume control by keeping the Grado closer to your listening spot?
As I mentioned in the email, if you don't like the quality of the audio from the Yamaha headphone out, all the Grado can do is amplify that signal. And you will have to balance two volume controls.
I have no idea who makes a "high quality" cable like that, because it is an unusual requirement. DIY is your best bet, or just a 1/4" TRS to dual female RCA jack adapter (Radio Shack 274-893, shown below) and a long pair of standard RCA cables, or a 1/4" to mini TRS adapter to a long mini-to-RCA cable.
