I think there are some capacitors missing in the last design. I would imagine there would be considerable supply modulation fed back to the first stage so I would recommend you add some sizeable capacitors (>100uF) between the input stage supplies and ground.
Current limiting the supply in a class AB amplifier will hugely increase distortion, especially at high volume levels, if you need to limit the output power it would be better to soft clip the input signal or increase the negative feedback to reduce it's gain. And no, you can't squeeze 500W out of a 260W transformer! You can't even get 260W of audio out it when you take into account the circuit inefficiencies.
Brian.