The standard system in a 182 is 40w RMS.
Should this read 4X 10w rms....
Maybe 4X 15w rms whilst playing a '0dB' 50hz test tone....
Distortion is apparent due to the amplifier within your Head Unit running out of steam mate....
A little snippet from a previous thread I did....
1, the average headunit is fused at 10amps, these 10amps at any one time are supplying;
cd laser
cd drive motor
cd laser carriage
unit illumination
flashy graphic display panel
and eventually the internal amplifier
When playing a full range signal the internal amplifier will require more power than it would if it were say playing a signal high passed at 100hz, so to translate this into basic terms, the more bass you ask your headunit to play, the more likely it is to distort,(this is due to voltage drop in the headunit, not poor speakers).
A simple test;
Play a dynamic cd, gently increase the volume until you can hear distortion, now lower the bass settings on your headunit and the distortion will stop, now increase the volume again, it should play at an increased volume without distortion.
A speaker can only reproduce what it recieves, if it recieves a good strong signal, it will reproduce a clean clear sound, if it recieves a weak signal thats ragged it will reproduce poor ragged sound,(distortion). the higher excursion that is required for lower frequencies demands a stronger signal,(which is why sub amps have higher rms ratings), and unfortunately headunits just don't have that kind of power.
When selecting a new headunit you should take into consideration exactly what you require from it. in most cases if you are fitting a headunit and using it to drive stock speakers you will notice an improvement in sound quality and output level, however if you decide to upgrade the speakers at the same time you may find its not as loud or warm,(bassy).
Most people when they detect distortion in the stock audio system run down the local audio outlet and purchase uprated speakers, then discover that it still doesn't sound any better, so they then assume the problem is that the headunit is still too powerful for the new speakers, when in truth the opposite is correct, the headunit is not powerfull enough.
HTH mate....