Re: PCM vs. HD audio codecs
In theory, there should be no difference between PCM and the DTHD or DTSMA. I wonder if the problem is the ProLogic processing that the receiver is doing? I'm not 100% but isn't ProLogic (even IIx) a matrixed, derived surround mode? I am wondering if your receiver is taking the discrete 5.1 channel sound and matrixing it, likely downsampling it, to process and derive the extra 2 channels. If I am correct (a big "if"), then you are getting 7 channels now, but lower resolution (maybe 16 bit, 48Khz or so), and NOT discrete channels.
If that's the case, then I would stick with straight PCM 5.1 channel or DTHD/DTSMA without engaging ProLogic IIx - you will maintain the higher resolution that way and also maintain discrete surround rather than matrixed. Your earlier post made it pretty clear that you like the sound of the lossless codecs - I would postulate that the vast majority of time, 5 discrete channels are better than 7 matrixed channels. Besides, if you use PCM without Pro Logic, then when you DO run across a disc encoded for 7 channels, you will get all 7 channels. Right now, you may be trying to force usage of those extra 2 channels to your detriment.
And that's really the answer - the reason you don't have 7.1 channels active all the time w/ Blu-Ray is because most of them are currently encoded for 5.1, and forcing ProLogic processing kills the discrete channels and *may* downsample the higher resolution bitrate from PCM.