Maybe the speaker crossover components (inductors, capacitors, and resistors in particular) are acting like the antenna in addition to the driver coil(s). I doubt if the crossover is shielded.

Ideally, with an amp hooked up and powered on, the dynamically low output impedance of a solid state amp (dynamically low because of the use of feedback to control the output signal, this feedback decreasing at higher frequencies, raising the amp's output impedance) shorts out the siganal picked up by the "antenna" element(s) in this case. In the real world, often tweeters (and midranges too) have crossover components in series between the driver coil and the output of the amplifier. These components will lessen the amplifiers ability to get rid of these unwanted signals.

With the amp off or disconnected, the "antenna" sigal is not attenuated.

Paul

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the 1derful1