How do I know if I need a sub woofer?

When you don"t have one.....or two.....

I guess if I get a sub or two my quest for a very good sound system will be over...thank god. 

I have owned many speakers over the years. There were a few that did not benefit from subs, but they were also cheap, off the shelf, great for dorm parties, speakers.

With quality speakers I believe most benefit. My full range Acoustat 1+1 speakers are really wonderful on their own with jazz, solo, and small ensemble performances. However, with orchestral, pop, or rock they are just OK. Add a pair of subs (originally Vandersteen, but now Rythmik) and the details explode. It seems to me that hard to drive speakers really open up with subs and become more musical when the bass duties are removed. I use an SPL Crossover (active) to limit the signal to the mains.

I have heard some amazing full range speakers that do not have subs. Surprisingly, many of the owners of these great speakers frequently add subs and are amazed at how much better they are. Subs need to be properly dialed in to make a difference (in my opinion both low and high pass crossovers provide the best benefits). 

Ultimately though, the end user has the final say. We are really here just to enjoy the music... I hope.

@soix, +1

@lanx0003, I think you need to wind you neck in, you do not know what you are talking about.

@OP, it's simple. All rooms regardless of size or shape need subs. It's not a case of it depends on the room, depends on your speakers or depends on what you listen to.

All rooms will have modes which means you have problems. This is an anavoidable scientific fact. Anyone who doubts this needs to do their homework.

What can be done? Well lots. Install some room treament. Have a look @tcutter's room as a fine example. Even with the extensive treatment seen in the photos and the admirably smooth response including envious RT60 performance subs will be needed. I say again subs will be needed. 

There is no other way to avoid peaks and nulls. If you are serious about your sound you should embrace measurement and fix the fixable problems. Most people now know about REW the free download so spend the equivalent of a dinner for 2 and grab yourself a mic.

The subs should not be ported nor have passive radiators. Get sealed subs and insist on having variable phase to make dial-in easier. REL subs do not have variable phase only 0-180 phase flip which renders them very difficult to locate and properly dial-in. If people have 0-180 phase flippers I often turn down the job. 

One of my subs is a REL and I have hauled this thing all over the room and finally found a spot that works reasonably well. Now add to that ports which are additional bass sources that resonate at only one frequency and can't be tuned and you are adding problem upon problem.

Somebody here said their sub is too slow! No such thing as a slow, boomy or one-note-bass sub. It's your room acoustics and poorly set up sub/s  If there are peaks 20dB louder than the average SPL then it will obviously take much longer to decay and guess what? You get to hear the complaints mentioned above.

It is simple guys, try reading stuff by Welti, Dr. Earl Geddes who wrote his thesis on this very topic and Toole, though I don't agree with everything Toole says. Avoid mindless ill informed posts from the likes of lanx and his ilk if you wanna move forward.    

lol