Audio Systems: amp/hu speaker wireing, rms output, amp power

Question
when hooking up an amp 4ch 50watts a ch to the interior speakers do i disconnect the head unit wires from speaker and only run speakers off the amp or do i use head unit and amp power.....if h/u is putting out 2v and amp has gain from 200mv to 4v should i match h/u output 2v? maker of amp said turn it up to 4v??

Answer
You should never run the headunit [and the amp outputs] to the speakers at the same time. So, just use the amp outputs.

You should match the amp to the head units' output, but in most cases, you will get the best results setting the amp to slightly more than 2volts so you have the ability to use tone controls/loudness without getting distortion since these will change the headunits' average voltage.
The idea is to never have the amp see more than the headunits' average rating. Less is better (as long as you can get full, clean, RMS output with the headunit cranked to 90-100%).
Once it is set correctly, you will have virtually NO distortion at full volume.

JM