Originally Posted by kbarnes701
It will be interesting, if Roger comments, to see if there is any scientific basis for what SB and I are hearing.
There is a basis for the differences you hear between DSX height and PLIIz. It breaks down to this:
1) Derivation of height signals: In DSX, it comes from L/R channels, which are active virtually all the time. In PLIIz, the signals are derived from the surrounds, which come and go, and are often silent.
2) Treatment of the height signals: DSX runs them through room reflection modeling to create early reflections. PLIIz does none of that.
3) Treatment of surround signals: DSX decorrelates the surrounds and drops their level some 3 dB. Not sure if these are related--in that maybe the decorrelation processing needs a few dB headroom, but if that were the case, it should have been compensated downstream, so we have to conclude it is intentional. PLIIz does not do any of this to the surrounds.
Taken together, the results you report are quite understandable.
When I mentioned about the DSX wides adding 4-ish dB extra bass to L/R sounds using the Onkyo TX-NR1009, folks (maybe it was you!) in the Denon 4311 thread checked and reported no such boost at all, maybe a couple dB reduction if anything, which suggested that the AVR makers have some influence over the results beyond core Audyssey definitions. Maybe that is the case, too, with the surround -3 dB levels, but so far, the anecdotal evidence has not revealed that being in play. It's very easy to confirm, though. Take a test disc with steady noise in a surround channel. Play it with DSX and PLIIz on and off. Read results on SPL meter. That's how I confirmed it.
Some might opine that 3 dB difference is not much. In an absolute sense, that's right. But when we are talking about the balance across several speakers, it can make a huge difference in soundfield perception. Decorrelation plays a further role in perception especially of transients. Not sure why it is imposed in DSX. If I had to guess, it would be that it helps better blend the surrounds with the DSX-processed front channels, which now have a layer of added "stuff" in the name of expanded spatial impression (but which brings along a dose of decorrelation). The native surrounds perhaps sounded too pointed in contrast, so decorrelating them makes them sound more like the fronts. But all of this comes at some price in detail preservation.