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.
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I had the similar issue with my AV receiver, except that it suddenly developed in my LF speaker.I ignored it for a while, but within a short time more audio problems developed.
I ended upn shipping it back to the distributor for a warrnty repair - some of the circuits had been fried and new parts and circuit boards inserted as replacements.
Speculation is that a sudden power surge (repeated) caused the problems.
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?
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.
It could be a ground loop issue, meaning that you may have a "dirty ground" feeding noise thru the amp. Have you tried a cheater plug to lift the ground and see if the noise vanishes? To be clear if your amp has a 3 prong plug, try one of those cheater plugs that a 3 prong plugs into but only 2 prongs plug into the wall socket available at most hardware stores and usually gray in color. About buck fifty. Another path of investigation is try disconnecting the center IC between the pre/pro and the amp. If the noise is still there then you know it is coming from the amp. If it goes away then start tracing it backward thru the pre/pro and source components. Do you have anything plugged into the 6 channel pass thru on the Krell. Pull the center IC on that
and see what results you have. Once you determine which component is generating the noise, try the cheater plug on that and see what comes of it. I have had similar issues and it can drive you nuts. I chose not to leave it as I figured something is causing it. It could be as simple as a speaker cable crossing an IC or Power Cord etc. Good Luck.