I've installed a solid state rectifier now. This improved the sound further especially on the low end. Output power is almost 6W now. I have to listen to the amp more to decide if I'm happy with the sound now.
I've added the voltages with the solid state rectifier to the schematic in blue. I've also checked the resistance accross the choke and measured 85R.
The voltages are still lower compared to the A12. Is this caused by the power transformer?