The Promethean branded stereo audio amplifier is a Class 2 double insulated item, the item is marked as such with appropriate labelling. The earth connection provided by the IEC mains is not a protective earth, merely a functional one. The reason for the functional earth is to prevent any potential HF instability in the amplifier should all the audio inputs to the amplifier also be floating (i.e. no earth) . The earth connection to the amplifier is not connected directly to the metalwork nor the amplifier circuitry but it is connected to metalwork via a 4n7 class Y capacitor. The amplifier will work perfectly satisfactorily and safely without an earth but under some adverse operating conditions could potentially produce some HF instability.
For PAT testing purposes the amplifier should be tested as a Class 2 device.