A squeaky "zoom" sound (similar to the sound of an overvoltage or capacitor discharge) can be heard at the REC OUT when the AMP1 is switched off. This sound also occurs when the MASTER is turned all the way down. The SPEAKER OUTs, on the other hand, remain silent.

This noise is normal and nothing to worry about. At the SPEAKER OUTs, the protection circuit mutes the signal when the power is turned off. Otherwise, a noise would also be heard here, in this case a loud "pop". On the other hand, nothing is muted at the REC OUT, which is why the noise described above can be heard here.

It therefore makes sense to mute the channel strip of the mixer/interface/monitor/in-ear system to which the REC OUT is connected when switching off AMP1.