Low level noise from spkr when nothing is playing

If it's only audible when your ear is close to speaker and it's not volume depended do nothing and leave it as-is.

Is it possible you have the gain cranked up on the center channel and needs to be adjusted? A line conditioner will not solve that problem if the issue is only the center channel. Is this the first time you've noticed this, or is this a new issue? How long have you owned the system? Any new cables, or any changes made to the system recently?

All of the pieces are new to me. The center is not cranked up. It has been there since I have owned the pieces. I can only hear it when I put my ear up to the speaker.

ground loop. Lots of threads. Had the same thing in my system.

Agree with Marakanetz. If you can only hear it if you hold your ear up to the speaker, then it doesn't really matter.

Off hand, I would try switching the preamp feed so that the center channel amplifier input is coming from a different output of the preamp. If it switches speakers, then perhaps it's a noisy channel from the preamp. If it doesn't change, perhaps its the amplifier center channel.

But honestly, what does it matter? You can't hear the noise except in a situation that is not normal (ie. you do not commonly listen with your ear to the speaker).

For what it's worth, some gear is just a bit noisy and I could hear some noise with high efficiency speakers on quite a few amplifiers if I held my ear up. I just ignored it.