OK, found the first clue. The receiver UI seems to include quite a bit of "if you set this then I automatically change these other settings" logic. I was running in "surround off" mode, which by default bypasses all the DSP processing. If the mains are switched from "Large" to "Small" then the surround mode is switched from "Surround Off" to "Surround Off + DSP", ie the signal starts going through the DSP in order to perform bass management.
Damn computers, I just can't get away from them.
I still wouldn't expect this to make much difference because I was playing the CDs on my DVD player through a coax digital feed so the only effect on the mains content should be the DSP's implementation of the 60 hz high-pass filter which I would expect to be negligible at the frequencies which matter to imaging.
Then again maybe years of abuse have rendered my ears incapable of hearing anything above 80 hz and so I'm getting all my imaging cues from the really LF stuff
One important point is that the better imaging in "Large" only applies when I'm playing in two-channel mode. Once I kick in a surround mode that extends the stereo content out to the surrounds (PL-II, Logic7 or Neo6) the difference between Small and Large is not really noticeable.
