Subwoofer

the difficult part of subwoofers is not messing up the upper bass, lower midrange...

If you want helicopters in your listening/TV room, a sub is wonderful. I have experimented for years and prefer no sum except when required for extra bass impact. All salesmen say that anything they sell helps.

danvignau,
I'll repeat what stringreen said, " You're not extending the low end, but making mids and treble better." That's has been my experience with my current set-up. My soundstage has opened up, mids and treble has better detail.

All speakers benefit from subs in varying degrees. Even speakers with capable sub bass modules at there bases. I agree with subs enhancing and increasing sound stage and all frequencies adding detail and putting more air between instruments. And the more room modes you can activate the more evenly distributed bass and flatter response becomes. But don’t expect a sub or multiple subs to perform miracles if room acoustics are inhospitable, thats at least half the challenge of acquiring well tuned bass thats tight and natural. Another point worth mentioning again is trying to practice some resistance its much too easy to introduce too much bass. Often listeners will go overboard on gains then wonder why they aren’t happy with end results. Just because its there doesn’t mean you have to use it. Less gain is usually more in terms of over all sound quality. Your less likely to start burying other frequencies or pushing the listening environments capabilities or carrying the sub nearer to its distortion levels where it starts becoming more obvious. One more reason I prefer using more than one sub and as many as 4...5..6?? If you have the space to go crazy I say do it.

You need to understand that a speaker can give it's lowest frequency when you play at loud volumes. When you play at normal volumes, the speaker does not go that low.

Subwoofers go deeper at lower volumes, this is a big advantage. And now comes the main part, you can point out low frequencies from 80hz. 

When you use a subwoofer with an extreme fast driver ( ceramics/alluminium) you can use it till 140hz with ease. This is including acoustic measurment. 

The influence becomes even bigger. It can make instrument and voices more palpable. You even hear more details when you compare the same music without a subwoofer.

Even stage depth and width is expanding with a subwoofer when you are able to use it the right way.

Most subwoofers are to slow in response, so you can't use them over 100hz. In these situations you are better of without a subwoofer.

But.....in 2016 it is possible to create a higher level in sound wiith a subwoofer than without. And yes we are talking only about stereo use.