Actually the PS3(toslink) vs HTPC(coax) was compared as AC3 transports. Material was POTC.
I do find a difference on PCM sources into the G68 on coax. I always run soundcards Bitperfect if possible and there are differences IMHO. Better clocks in the source gives the G68 a better time..
Better soundcards give better dynamics and more punch.
Weiss Minerva Firewire external card is the best SQ SPIDF sound card I have tried in my system. I think it would give the HDI Dune with HD621 a tough match, though I don't have it on hand as I will not put any money on SPDIF sound cards atm. It's HDMI LPCM audio all the way now
I have dreamed of the SQ of the M 808 from mediaclients/servers and the HD621 put me a bit closer to that goal.