How do I know if I need a sub woofer?

I think well integrated any sub is worthwhile, the problem is the "well integrated" part is often a beast to achieve.  

So, to quote the Bard, "what dreams may come must give us pause and make cowards of us all." 

Start by measuring, and considering your room first.  An untreated room may have far too much treble already.  The room may be giving you a lot of bass in the wrong areas.  Room treatment and careful EQ may be a better starting point.   Use the AM Rock room mode simulator to help en sure your speakers and listening position are in the right spots.

If considering a sub, where will you put it?? The room simulator will help you find  an optimal location. 

my point is, as audiophiles, if we need a sub, we subconsciously know it, and don't sleep well.

buy a stereo pair of front facing subs, no ports, and locate them adjacent to your mains, to you gain more lows and get the directionality contributing to imaging that the overtones give.

As long as you own good speakers that (1) comfortably reach down to the low 30Hz range, whether floorstanders or bookshelves, and (2) have reasonably wide horizontal and vertical dispersion, you don’t need a subwoofer. I’m talking strictly about music listening. The problem with subs for music is that they are very difficult to blend seamlessly with the main speakers, especially in terms of pace and texture.

Movie watching is a whole different story.

Just be careful reading measurements...lots of speakers rated -3db at low/mid 30's, are doing that measured at very low volume and very very close to the speakers...and are not even close to that at 10 feet away at full listening volume...that's why a pair of subs can make a huge difference in many ways..and while 1 sub is difficult to set up, 2 are much easier...

@jl35 +2

Mike