On Friday, kswanson and I were in Stereo Design`s (San Diego) Meridan 8K demo room. We used the modded Oppo for some 2 channel CD listening, hoping we would hear the apodizing difference made by the HD621.
The Oppo`s modded three SPDIF digital outputs were routed directly to a G61R (old version, upsampling but no apodizing). We listened to a CD with a known harsh and edgy vocal track without upsampling turned on in the processor. We both thought it was pretty edgy and not easy to live with.
Then, the G61R setting was changed to output high res. The yellow bar now appeared on the 8Ks. The sharp edges were moderated. We both heard some improvement due to the processor`s upsampling. No apodizing, yet, of course.
Then we switched the output to the Oppo`s HDMI into a HD621 and then via Smartlink SPDIF into the G61. We heard a significant difference/improvement in the vocal track. The harshness was mostly gone and voices and instruments seemed to have more “air” surrounding them.
We could and did switch back and forth between the two signal paths. We also listened to some more complex classical tracks including a Reference Recording HDCD. We left convinced that the HD621 does the job as advertised.
Our experience confirms reports by other HHers that the Oppo is preferred using HDMI with the HD621. I am convinced and will be spending some $ next week. We are not claiming we performed an unbiased, scientific double blind test or anything like that. We hoped there would be noticeable, audible improvement. We think there was, that`s all.
For the record, the harsh vocal was Rufus Wainwright`s second track of his Want One CD, song titled I Don`t Know What It Is. The classical track was Respighi`s Adagio on Mischa Maisky`s CD titled Adagio. And the Reference Recordings track was Copland`s Fanfare on the Copland HDCD

Roon, Technics SL10 turntable, Meridian 218, Jolida tube DAC, Nuforce STA 200 Amp and passive Swallow Acoustics ALS2 speakers