What would cause my cd to play at very low volume even with the amplifier volume knob at 12 o'clock, while my phono played at a level appropriate for that volume setting?

When I turned off the amp and sources to alternate ICs from one input to another, on restart levels were equal. Turned off amp and returned ICs to original inputs. Started up again and everything was fine and has been since.

The amp is a Cary SLI-80. All tubes are new. ICs are clean and tight. On initial start up I didn't allow much time for the amp to warm, but would that cause the volume to fluctuate from source to source?

Thanks for any insight.
That's enough time, IMO. Maybe not perfect sound, but levels should be stable by then. I'm guessing you had a bad connection somewhere in the chain, that was resolved when you unplugged and replugged ICs. Just a guess.
Are you sure that there's not an internal digital or analog volume control in the Cary? Many CDPs have them so that you can eliminate the pre-amp. Control is usually only possible via the remote. If there's a Volume button on your remote, try pushing the Up arrow.

I couldn't duplicate the effect.

Since then, I've been thinking: I always use the knob to control volume, but the last time I listened, I used the remote (because it seemed like a cool, unusual thing to use). Dcstep, could I have activated then deactivated the volume control in the Cary by using the remote? Would I have reset it by switching ICs?

Am I throwing darts blindfolded?
If there's a knob on the Cary and a volume control on the remote, set them both at the max while turning down the volume at the pre-amp. If that doesn't fix it, try rebooting the Cary by disconnecting its power cord for 20-seconds or so.

You'd have to have really horrible ICs for that to be the problem, so I doubt that's the issue. Just make sure that they're fully connected and seated.

Oops, I've been saying "Cary" as if you have a Cary CDP. If not, substitute "CDP" for "Cary" in my previous posts.

I understood you from the first, Dave. Thanks. You too, Swampwalker. I just unplugged the whole works and plugged it back in. Nothing unusual happened. Everything works fine. Just a weird experience that had no explanation. Haunted, maybe.