How do I know if I need a sub woofer?

The other question would be, should I get sub(s) or use that money to get better speakers...

Pair of REL T9x, a single sub has never worked for me.

Understood.  Empirical evidence has suggested dual works better than one in terms of better elimination of nulls caused by room mode but it does not resolve the pace issue.

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...