Sadly I don't have experience on the HD621, but I hope I can add a little something to this interesting thread with my perspective on the lossless-v-lossy debate.
I'm using multi-channel analog to listen to lossless from my Sony 550: what I've found (on what I would call my entry-level high end system LOL ~ Does Denon THX qualify me at the bottom of the high end?) is that the best DD & DTS soundtracks continue to delight me, but that the better lossless soundtracks have an ease to them. They feel very organic. It is not night and day, but you can tell - even when you're not in Meridian land!
In a way I feel the same way about the picture - a well upscaled, top-notch DVD will make you forget you're not watching HD (I'm thinking here the big multi-disc versions of the Lord of the Rings, The Star Wars Trilogy, Pixar animations like Finding Nemo and A Bug's Life among others) but the step up to blu-ray brings an ease and authority and promotes a relaxation in the viewer like your brain doesn't have to work as hard at watching. I hate terminology like 'wow that really pops' as I don't want to be bludgeoned, I want to be seduced. And the best blu-ray is very seductive, IMHO.
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