Seventh Circle Audio

Ive gone back to the schematic to better understand the circuit design and I pretty much have it figured out with the exception of U2. What is the function of U2 in the circuit. I know its part of the gain control and is probably some sort of DC correction circuit but could you give me a little insight behind it?
Thanks

U2a maintains 0V DC at the inputs of U1, U2b maintains 0V DC at the output of U1. See the THAT 1512 data sheet for more details.