It looks like a very dangerous project to me, firstly the claim "this circuit is exclusively for amateur use"
Amateurs should not be experimenting with mains voltages, especially when they are not sure what they are doing and could kill themselves.
5 kW at 120 or 240 Volts would also cause fuses to blow and 2 ohm speakers to burn out.
It looks like better to stay away from this project and certainly don't get conned in buying it from whoever is trying to sell it.