Thank you, John. That makes sense as I was beginning to suspect that the problem did not happen with raw files from the 6D shooting in raw only. (The Rebel XS still seems to have a problem with the direct raw import, but that has been documented on other forums. I can open them in Photoshop, however).
On a related not, if you are the John I think you are, I read your excellent article on migrating to LR. This saved me many headaches for everything else, so thank you, thank you, thank you! Having read the article again—and with what you just suggested here—I am thinking of starting over with an export of raw originals in from Aperture to one directory tree (raws) and the jpegs to another tree (jpegs only). The TIFFs with baked in adjustments came across just fine, so I won't tinker with those. This will take a few days, so stay tuned; I'll let you know what happens.
Cheers,
Paul