Create a new source by clicking the '+' at the bottom left of the app.If you go this route, here's what you need to do:
The app comes bundled with dozens of filters, and the one that works for me is the AudioUnit Effect Graphic EQ. However, you can choose to simply hijack, and filter the audio, and not record. You do this by 'hijacking' the audio, and then recording. It's designed to record audio from any source, including system audio. I decided to buy Audio Hijack, from Rogue Amoeba, which costs $32.