How do I know if I need a sub woofer?


My system at the moment is not important as this question would be relevant regardless of of what I am listing to at the moment. 

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Depends on what your listening objectives are.  Home theater- subs can come in handy.  Mostly 2-channel music, you would be better served by using speakers that can supply ample bass bandwidth.  

The sad reality is that unless you have full range speakers (usually very large $) AND sufficient current (not just watts) to drive them you need subs.  Sigh. 

You will know, if, you are not happy with the low frequencies your speakers put out.

First, you want at least two subs, and second there are very few systems that wouldn’t benefit significantly from subs.  Anyone who tells you differently hasn’t heard decent subs properly set up and dialed in.  Not only will your in-room bass improve but also imaging and an expanded 3D soundstage.  Once you listen with good subs you won’t wanna listen without them.