How do I know if I need a sub woofer?

buy this, enjoy it? DONE! Sleep well.

https://store.christianmcbride.com/products/christian-mcbride-edgar-meyer-but-who-s-gonna-play-the-melody?srsltid=AfmBOoplzVr4lJta9Vyk8pj2s6H5GbaJdzpJFFdCzAH_byXcoVAySgeo

Edgar usually plays Cello, but it’s two Upright Bass this time (and they both play a little piano).

This, similar is almost 50 years older

https://en.wikipedia.org/wiki/Double_Bass_(album)

My friend has a situation where he often wonders if he needs a sub, especially after listening here, but I usually conclude after listening at his place that he really has enough in his listening space. 

I brought this LP to his place, we thoroughly enjoyed it, I’m sure he slept better that night.

Sometimes, just because you can doesn’t mean you should, it’s a blessing to be happy with what you have.

i.e. Spikes below speakers. I didn’t think I gained, and in fact lost the ability for alternate toe-in, I gave my spikes to the same friend many years ago, he still uses them, I wish we could alter the toe-in for two listeners like I do when he or others listen here.

https://www.audiogon.com/systems/11516

some listeners are never going to like subwoofers...

Besides extending the bottom octave of your system, adding subs will also improve soundstage width and depth. 

There is some spatial information in those very low frequencies.

It is most noticeable in orchestral classical music, but all music will benefit in this aspect.  

The only way to find out is to try one and see if you like it! What speakers are you using?

to hear and FEEL pipe organs as they were meant to be heard and FELT, you need to be able to reproduce cleanly down to about 16 cycles per second. precious few stereo speakers can do that trick. a pair of subs is mandatory for all us pipe organ lovers.