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@thor I'd be willing to bet its the powersource. Your speakers are likely using a switching power supply running at 4 khz.. the reason is a few 1) thats typical for guitar amps 2) the harmonics are typical of a square wave, which means your seeing RFI from a PWM like in a switching power supply.
So my top guess right now with like 80% certainty is that it is the power supply to the amp (if its a wall wart its that, if its a straight plug its internal).
@freemo you could be right that it's switching noise. the power supply is internal. these plug straight into the wall. i'm wondering if this can develop over time, because it would be pretty bad to send them out of the factory like this. and i'm noticing that one speaker is making more of the noise than the other.
