If the AMP1 is connected to the power supply via the standard power cable and to a loudspeaker cabinet via a speaker cable, there is a quiet but audible hum in the cabinet itself, even when the AMP1 is switched off. This is perfectly normal.

The mains switch of the AMP1 is located in the secondary circuit. As a result, when the amp is switched off - as with many modern devices - there is no proper disconnection from the mains. Strictly speaking, the AMP1 is then on standby. The transformer therefore creates its magnetic field even when it is switched off, which then spreads directly into the speaker and becomes audible. Incidentally, a similar phenomenon occurs with many other tube amps.

If this bothers you, you should use a switchable power strip with which you can completely disconnect the device from the mains by switching it off when not in use.