An HD621 will do nothing for the image. You'd need a scaler for that.

I'd always used a DVDO (masquerading as a Meridian 2351) but I sense that they've been left behind and certainly the Lumagen video scalers appear to have more than surpassed the DVDO range.

For audio the HD621 will allow you to access higher resolution audio. This can only be accessed from the HDMI (or modded 3x SPDIF out players) due to bandwidth issues. This will improve significantly your audio over and above the standard output from your Dune.

For video players the Oppo products generally win the best plaudits for spinners (and value for money) but have more limited streaming capabilities. The Kaleidescape players spin and stream well.

Sooloos, Roon, G series (with LPS upgrade), 800 series (with LPS upgrade), HD621, AC200, Prime and PS, DSPs old and new.

Purdeys, Moke, Leica, 'Blad. Only the best.