Could be a tube, or, could be the infamous balance control. This series (as well as the 5.1 and 7.1 had really noisy balance controls, manifesting itself in the type of noise you are describing. Try giving the balance control a good cleaning with a spray cleaner. The Counterpoints are really sensitive to microphonics as well.
Does the noise increase with an increase in gain, or is it at the same level regardless of volume setting?
Does the noise increase with an increase in gain, or is it at the same level regardless of volume setting?