Hi sptrout. Although I am loathe to post in any thread started by the OP, I have to say that I do really enjoy it when the old skool AVS 805ers post here, including yourself and riverwolf. Now if I could just get BB over here too, that would be a massive brainiac influx.
Regarding the portion I boldfaced, I do believe the xx5 series can have Flat curve, but only on THX modes. I believe Chris has stated that in fact the Re-EQ setting will not alter at all the Flat curve (and that's even when Re-EQ is a THX setting to begin with).
The problem with Onkyo however, so far as I believe for most discerning consumers, is that you can no longer get Flat if outside of a THX mode. So, that includes stereo as well. I had a PM conversation a couple of years ago with nibhaz, who hasn't posted here in quite a while. It appears I have deleted the PM, but I still found what he told me he does with his version of "stereo" mode, and I'm pretty sure he doesn't mind me sharing it. I've never tried it myself, because my HT is always using a matrixing algorithm (except in the case of native of 7.1 tracks).
(edit: regarding RE-EQ, that setting CAN be made on top of the Reference/Audyssey/House curve. Yet another thing Onkyo/Integra owners need to be aware of.)
I think that overall, people who want to learn more about what Audyssey is doing, they should look very specifically to the brand/model, be very aware which target curve they are using, what the selection even is to begin with, and that is also respective not only to model/brand but listening mode.
There is also the Front curve (Marantz?), as well as the NAD/PSB curve. There's probably more for all I know.
I am using almost exclusively PLIIX on my 805, [strike]which means Flat curve, regardless of RE-EQ setting[/strike]. Excuse me, I meant I always have the Reference curve. Re-EQ remains off (I believe as default too?). I am going to try THX modes again though, as my new screen material sort of provides a natural roll-off on the center speaker.
I almost think the varying manufacturer implementations are even more confusing than the nature of Audyssey! If and when people complain, I'd like to know what settings/curve/model they are using, sometimes.