Actually, I was planning to reply to all the very helpful suggestions. I found the answer yesterday, and this is my first chance to get back to the forum.

Turns out that Mr Coyote was the culprit. I have Realband set to use my Creative Audigy (A) synth, and BB was set to Coyote. When I set BB to the Audigy card, the problem vanished! Sound is, of course, much better (not that I'm going to leave well enough alone). With Coyote, the metronome count-off sounded like a couple of pool cues being hit together.

So, I am happily at work recreating my library of original stuff, lost 2 years ago.
Thanks to everybody who weighed in on this issue!