Thanks for the additional thoughts, guys.
The problem shows itself when using the same paper from one box, so it can't be the paper. The whites weren't grey or brown by any stretch -- just not as bright as when I printed with fresh chemistry ("non-fresh" meaning more than a few hours old). Blacks in the image were still coming up fine, so I don't think it's a matter of exhaustion.
I haven't printed in a month or longer but will soon be doing so again and will try to hunt the problem down.