@crion,
Interesting enough I hear a similar difference when playing CD using the optical and using the HDMI through the Octava. Using HDMI the soundstage is much wider and the bass sounds more controlled and reinforced. It seems that this difference has nothing to do with the upsampling at all ... just for testing purposes, you could disable the upsampling in the HD621 and see what the result is. I bet you will hear the same difference.