You need to divide and conquer. Remove tubes and try. If it blows, bypass VRM. If it's OK, it's the VRM.
you should probably make a light build current limiting device to save Your amp.
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Essentially, it is (as the name implies) a single light bulb in series with one side of the AC line. It plugs into the AC outlet, and the equipment being tested is plugged into the limiter. If no short is present, the light will stay unlit or glow dimly. If a short is present somewhere in the unit, the bulb will glow brightly, as current is drawn through the filament. The higher the wattage rating of the bulb, the higher the current limiting capability. Use a 40 watt bulb.