I Changed 121k to a lower (2k) value and 464k to a lower (8.2k) resistor to alter output level. Op Amp used was OPA2134, the 5uF is an audio grade one.
This amp was built and works, it needs running in a bit as it sounded as bit thin at first and after 10 mins is sounded great. I use it to boost the output of my phone, so I can use proper headphones.
Note: The circuit shown is just one channel.
