I have two stereo setups with a sub each. From a bit of experience in the production world and as a listener I would like to make a pros and cons list. I think it might help making a choice since a lot is in the gray area of decission making in personal preference cases.
Why to purchase a sub
1) a little extra reach at the bottom edge. It makes bookshelf speakers sound like there are floorstanders.
2. Some music utilizes a subwoofer.
3. Great addition for Jazz, in the sense of being there.
4. Atmospheric movie effects.
I use them a little below the bass extension, to a point that when you turn on the sub there is slightly more presence. But you don't really notice that there is a sub if you don't know there is one. With the right combo in volume, and bass extension point it will do well allround.
against
1)Don't only do it for the explosions in movies. About 90% of movies and series focus on dialogue, so a good midrange coverage is by far more important than deep bass.
2) Also keep it in mind that when your room conditions are ok and you have found the ideal setting for a reproduction of that typical boomy cinema sound, it might be totally off for music.
3)The versatility of a sub is both pro and con at the same time. Dialing up the sub's volume for one track to enhance the atmosphere might be a bad idea for the next track. The weighed settings for a little extra support are usually preferable.
4)Don't really do it for music in general. There are exceptions, but most music doesn't even really utilize a sub. There are also examples of tracks with bloated bass above some sub's frequencies. A sub doesn't compliment those well when you are a bit free wheeling with the sub's volume.
5)A sub requires an amp with a dedicated sub out in order to extend and make the extension dial work and fine tune. Only get a sub if you are ok or prepared to change an old amp. Otherwise don't do it.
You can't just add any sub to your shopping cart. Subs simply require "nerding around" on specs. You 'always' need to know what loudspeakers you stick to in order to find a matching sub. Why? Because you need to find the bass extension point of your loudspeakers in the manual. Your sub needs to cover a (by preference) wide spectrum of frequencies below this point. Otherwise your sub just doubles your woofer output and anyone will kind of regret. It has cost me days until I finally found a match and I am glad I didn't buy just one impulsively.
ow yeah.. for when you have a really small room and no space for a sub. The Arylic A50 (a really small amp) has this double bass setting for cinema. It might sound sketchy but it is effective.