[Herald1360]

The "store bought" amp circuit shows how it should be done for safety- there's an isolating mains transformer and the ground pin of what looks like a standard "kettle connector" is taken to the chassis and the audio input ground side. It also (interestingly) uses dc for the valve heaters presumably for hum reduction without needing to be particularly careful about heater wiring layout or voltage balancing.

To answer your safety question about the two prong mains lead- Yes there is the possibility of a dangerous situation if the insulation in the mains transformer were to fail or if a mains live wire were to come loose and touch the chassis. With no ground, all could still work but the amp and its signal input could become live. With the ground correctly connected such a fault would blow fuses or take out an RCD rather than electrocute you. In general, safety requires that there must be a minimum of two independent failures before any part of equipment can become live. This is normally achieved either by double or reinforced insulation or by grounding any accessible conducting parts. There is always the possibility here that someone has deliberately compromised safety to get round a hum loop problem rather than solve it correctly by wiring layout or using a suitable signal ground separating interconnection such as an audio isolating transformer.

To achieve the same result with your live chassis amp designed for 120V mains you would need a suiutable 230-115 mains isolating transformer to feed the ac supply to the amplifier. With this in place, then the chassis and signal ground of the amplifier can and should be connected to the mains safety ground.