When I say tinny I usually say it as “hollow and tinny” the way it sounds when you string to tin cans together and try to talk between them. On my Denon I could get a snapshot of what was causing this by copying over the Audyssey curve to the manual EQ and was seeing a -6dB to -9dB cut in the 125Hz -500Hz ranges. I came to believe it was a defect in the equipment, however I’ve heard of enough other people describing the same thing that I’m not sure about that.
So if someone wants to simulate what I was hearing from Audyssey on my Denon just use manual EQ to lower that those frequency bands 6-9dB and have a listen. Then try to tell me that it sounds right better with a straight face.
OTOH I’m not have that problem with my new Onkyo, however because that problem is gone I can now easily hear the high frequency roll off of the Audyssey curve. It kills the ambience in most of my music.