Thanks for the replies, good to have a solid answer on that!

With all due respect though Tyler, you are incorrect about speaker and line levels being the same. There are 3 basic levels used in any audio system, whether it be a car stereo or a recording studio. Those levels are mic, line, and speaker.

Mic level is the weakest of the signals, and is just what you described, and is also comparable to the direct output of a radio tuner circuit or the direct output of a CD Audio DAC.

Since mic level is so weak, it generally gets amplified up to line level, which can be either 316mV RMS or 1.23V RMS. You could compare this to the level of pre-outs on head unit. They're higher than mic level, but still not enough to drive speakers.

Then you have speaker level, which is obviously a signal loud enough to drive some form of loudspeaker, whether it be a headphone driver or an 800 watt sub.