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If this only happens at full max volume rotation, could be the pots wipers are going open circuit, and the input of the next stage (output line amp section) is seeing this open circuit and will be noisy. If this is the case and it’s fine when you back it off from full a little, then don’t worry about it.
I agree with the others that it's nothing to be concerned about. The cause could be what George suggested, or it could be the inherent self-generated noise of the input stage of the preamp amplified by everything that follows (any electronic circuit inherently generates a non-zero amount of noise), or it could be noise introduced into the preamp as a result of a ground loop between it and the source component (ground loops can cause high frequency noise as well as low frequency hum). Or it could be a combination of some or all of these things.
Contributing factors are undoubtedly that your speakers are more sensitive than average, and the gains of your preamp and power amp are a bit more than average.