One thing I have noticed though is I get sound from the rear speakers only if the low-pass filter on the amp is turned off.

If I turn it on, I get sound from the front speakers too but it seem to be low frequencies only.

So, anyone know how I setup an amp? The amp has got several buttons and knobs on the back, but I don't know what they do, and the "instructions" for the amp aren't much help either:

Link to amp instructions (PDF)