It is fairly well documented and, if you have ever used MConfig, this is not a big job. Several of us have posted how we did it at Meridian Hitchhiker forum.
Since you have an HD621, can you comment on the differnce it makes for sound? Comparisons get complicated and uncertain when you make more than 1 change. So let's make the simplest comparison:
1) G98 -> G68 MHR
2) G98 -> HD621 -> G68 MHR
Would going from 1) to 2) make a difference in sound? Or was your intent primarily to provide an HDMI switch?
I have the same set up you are considering Axle. I think it sounds great. I purchased the ATV specifically to stream via HDMI so I have never tried it direct via S/PDIF but I have streamed directly from my Macbook Pro via optical and the ATV-HD621 is in a different league IMHO.
Set-up isn't too difficult if you are MConfig literate at all.