professorhat:Just as an update to this, funnily enough, I had a party at the weekend and so was using mostly music streamed from the Mac through Airport Express (something I rarely do in normal listening scenarios). Setting the whole thing up before the party got going, I wasn't happy with the sound on the Onkyo's Stereo setting. I changed it to Pure Audio mode and, like a flash, it was instantly much better. My previous results were all from playing music from my Marantz CD player.
So it would seem the result is very much dependent on the source - which makes a lot of sense when you think about it!
Thats funny...I've had a similar experience myself. I always preferred the 905 in Stereo, however more recently it sounds clearly better in Pure Audio. I wonder if this is a 'running in thing'as in the past it always sounded a bit harsh in Pure Audio. Perhaps the Stereo setting somehow subdued this as it is less revealing.
The other factor is that I added a Clearer Audio Copper Line Alpha mains cable to the amp and CD player, which also had significant improvement in the sound quality. I also placed the Amp on a proper hifi rack, which gave it a clear lift. Professorhat did I read that you were experimenting with mains cables too. If so, that may be the answer. Perhaps the Pure Audio setting on the amp is more susceptible to the benefits / problems associated with mains supplies.