Receiver's Audio and HD Audio performance

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Simple.

Cost.

Why do you think the Onkyo 605 and 606 receivers are so popular? It's not because of their astounding audio performance (although they are good for the money). It's because they are cheap.

If you want excellent specifications, and I'm talking truly state of the art, expect to pay no less than $10k for your processing alone, and then add another $40k for 5 channels of high-end amplification.

What you can get for $500-2000 in a home receiver these days is truly incredible (my current Yamaha does so much more than my old DSP-A1, and cost me half as much), but manufacturers must cut corners to do so. Obviously, they are choosing the right corners as nobody is complaining, except for those combing over the spec sheets.

edit: Regarding the 0.05% THD, the manufacturers are saying they'll hit (say) 100W with 0.05% THD, they're not saying that they are outputting with 0.05% THD all the time. As you lower the power output, THD drops. Most of the time, when you're coasting along at a couple of watts per channel, THD would be nearly unmeasurable.

For superb THD figures, I suggest buying 5 Krell Evolution One mono power amplifiers. Each one is capable of outputting 450W into 8 ohms with under 0.01% THD. Small issue though, they cost around $25,000 each.