In my case it comes from experience. The older, pre HDMI Theta was just superb sounding in all formats compared to modern, HDMI equipped Onkyo and Emotiva models which I owned briefly. Even using the Onkyo as a preamp there was absolutely no comparison. The Theta was a far more musical and pleasant experience, especially from the mid to bass range.
The closest I got for DVD/BueRay with HDMI was an Oppo to a Halo P7. That also sounded really good. It was a long time until I had a processor (Anthem) which could compare in sound quality to the Oppo’s analog outputs.