Thanks a lot for your answers guys!
NYD seems to have removed all of his schematics. I like conductor's schematic, thanks!
Is there any chance someone could tell me what these three ICs are? And what should I do if I want to make it 2/4Pole switchable (probably add another opamp, but don't know where to start looking, links concerning filter design are also acceptable if you don't want to break it down for me). Also what is the simplest solution to balance/unbalance a signal? Thanks a lot everyone! Sorry for the mind-numbing questions, I'm not good in electronics (just good with a soldering iron for now).