2n3055 transistor specifications
The schematic gives much clarity. I understand, that you connected three 2N3055 in parallel as mentioned in the text. Paralleling of BJT without emitter resistances isn't guaranteed to give balanced current distribution normally.However, in this case, I think it's O.K., cause the CE saturation resistance achieves a balance.
I don't see any power or current notice in the schematic, and I basically doubt that it's capable to deliver 500 W in the present dimensioning. Calculating with typical 2N3055 current gains, you can easily see, why.
The pre-driver is supplied by 5 V and may source around 20 mA (3V/150 ohm) to 2N3055 driver. It has e. g. a current gain of 75 and sources 1.5 A, 0.5 A to each output 2N3055. It achieves e. g. current gain of 15 and 7.5 A output current. All three together carry 22.5 A. Neglecting transformer magnetizing current and any switching losses, and assuming only 2 V voltage drop for the darlington, you get 10V * 22.5 A = 225 W total output power. A bit more, than the reported 160 W, but actually, there are switching and transformer losses and a magnetizing current, too.
Furthermore, the 2N3055 may have a much lower current gain without being a fake product, completely within manufacturer specification. To be operational by design, a circuit must consider specified minimal current gain.