few questions about the audio from my 506

[sheedoe]

If your receiver cannot handle pcm via hdmi, what you may be hearing is the lossy DTS or the Dolby digital track. Dts has higher bitrate than dolby digital, so it sounds better.
That explains why you enjoyed the DTS-HD encoded movies more because they also include the core dts (1.5 mbps) sound track.
There's a noticeable improvement in sound when you step up to lossless audio. So my answer is 'Yes' to your second question.