I tried a little last night - imported some Rickie Lee Jones live material from a concert I attended - and on that material the 48K upsampled AIFF files were preferable. If I told the Mac Digital output to send 44.1 to my DAC insdtead of 48, the difference disappeared. I didn't succeed in determining if the 48K output was overall superior to the 44.1, and will leave that for later, because there is a question I find more important to address first.
The next thing I want to know, and I'm going to contact a mate at iTunes to help connect me with an Apple engineer, is how to get a TRUE bit-for-bit copy of a CD onto the mac and into iTunes - no upsampling, and no REsampling. I also tested files imported into iTunes at 44.1KHz 16 bit (Redbook CD standard, should be {skyquotes}THE SAME{/skyquotes} as the original, right?) versus files copied from the CD through drag and drop in the finder, then played back in iTunes.
These sounded very different. I am presuming that the drag and drop copy is the "correct" file, and that iTunes rejiggered the bits in the direct import. But that's what I want to find out. If I can make an exact copy of my CDs on the hard disk, that leaves the future open for resampling/upsampling or whatever intentional jiggering is determined to be advantageous without the prospect of losing something or the effort of going back to the original CD. But as poor as CDs are, I am loathe to change the material when I archive it on my hard disk - even if it's better some of the time, you know what I mean?