Hi
My advice would be as follows:
buy a smaller ( 8" or better 10") QUALITY sub ie good brand name ... mutant is a Budget brand !! You should find that a quality 8 or 10" will sound better than a budget 12" ... & last longer too!!!
The rule of thumb with ANY speakers is to ensure that the speakers are capable of handling about 50% MORE power ( MAX Power ) than the amp that will be powering them ... this should mean that you wont blow the speaker/sub as soon as you turn up the volume for more than a few moments!!
Also, if the amp is not powerful enough, then the speakers will never 'move' enough to give the quality of sound that you'd expect!!
Similarly, spend your money wisely on the Amp, as budget brands tend to exaggerate their output, whereas quality brands tend to be slightly better than their packaging claims!!
To give examples might
help:
Head unit max power 4 X 45w - each of the speakers need to be rated between 65w and 180w ... any higher than this & you should really use a small amp - usually this would be 6x9s at the rear!!
Amp max power 400w => sub max power 600w
Amp max power 800w => sub max power 1200w
Beware the various ways the manufacturers state Power Outputs on Speakers / subs ... MAX , RATED , RMS - this last one is the most accurate 'real world' measurement BUT you usually find that the MAX figure SOUNDS impressive & thats the figure people shout about ... BUT the MAX figure tends to be the power attainable JUST b4 it goes 'Bang' !!!
If you want to start an Install & do things right first time ... i suggest you invest in the Haynes ICE manual & read it cover 2 cover ... it's full of facts, figures & mega useful hints / tips which will save you many £££'s in labour, wrong items purchased / fitted, etc..
Hope that this gives you enough warning about the subject ... best off spending £20 on that book & do it right!!!
Gd Luck