What are these inputs for this audio power amplifier?
simulate this circuit &ndash; Schematic created using CircuitLab

Figure 1. Possible internal wiring.


The asymmetrical input would be signal and ground - similar to a guitar jack.
The symmetrical input would be for a differential signal - similar to a balanced microphone for low level signals or long distance. A transformer can be used to unbalance the input internally.
Earth loops can be a problem with audio electronics. Typically this happens if two pieces of equipment are connected to mains earth but also connected to each other via a shielded cable. The earth lift switch breaks this loop by disconnecting the audio ground from the mains earth. The metal chassis would normally remain earthed.


The only problem with my last point is that the ASYM jack isn't isolated from the chassis which appears to be metal.