Given HDMI output chips have their own clocks, I somehow doubt that changing anything prior to the HDMI output (which is all upgraders are able to do) will help.
I emailed audiocominternational.com regarding upgrading my Dune player.
They already offer an upgrade for the Popcorn C-200, and they said the Dune upgrade will be available shortly.
Apparently its the master clock they upgrade, which controls all the other clocks, and it reduces jitter dramatically from the stock clock.
I might get it upgraded just to see what differences i can hear, when feeding it into the HD621.
If the HD621 performs apodizing and reclocking before feeding to my 861v4, i wonder if the 861v6 re-apodizes the digital input a second time?