I was really bothered about the frequency response, so I decided to take some measurements.

I used REW (Room EQ Wizard), and got a very flat response for a direct cable.. but a (I think) quite bad for my mixer :(.

(blue is -48 dBFS, pink Phase)

I will try to get more adequate Op Amps, and maybe tweak the dc coupling. 

Also somehow I think I do not output enough volume... and I used 15 volts for that.... so maybe I will have to redesign it all?

Suggestions are appreciated!