Personally, i think das PCB has a lousy layout. But it is small - advantage.
The different voltage comes from the LM337/LM317. Read their datasheet and google how they work. Some people like me have used resistance trimmers to equal them ( http://szcanyi.en.alibaba.com/product/307861290-50332208/3296W_103_trimmer_resistor.html ).
And where should i start... have you measured the DC-offset before connecting your headphones?