$100 says you have your input voltage set to "High".
If the input voltage switch is set to "High" you will have considerably lower output(sounds backwards but it's correct), only set to "high" If your using 8v outputs on your headunit. Set to "low" and then set your input voltage knob, "gain", accordingly.
Hope this helps.
Also, if your headunit is doing the sound processing set your bass eq to "off". When it's set to "off" your "Q", "center frequency", and I also believe your "boost" is defeated(non-active).