80 Hz LPF, 30 Hz Subsonic, 12 dB Bass EQ (if it does not blend well with the front stage, set it to 6 dB). For the Gain, set it completely off, turn up radio to the loudest you would play it on a song you know well. Turn up bass knob halfway, turn Gain until bass sounds great with the front stage.
As you listen to the system you may want to make Gain adjustments. Do not hesitate to fiddle with it or the bass knob. You will eventually find the perfect spot for your music preference.