Here's a diagram of the current setup:

This results in mixed audio going to the camera which (due to various settings he likes to use whilst performing) results in a very muddy recording which I can't really do much with in post.
I'd like to insert some hardware in the chain that allows me to pull off the clean vocals whilst still sending them to the Karaoke amp - preferably something that allows me to control the gain so I can boost/cut as needed.
Any suggestions? Thanks!