How do I know if I need a sub woofer?

It should boost my testosterone as well.

Sub(s) is / are panacea for adrenaline, body resonance, mood, hormonal harmony.

I added two JL Audio powered subs alongside my B&W 803 D3 speakers.

They transformed my system!

Bigger soundstage, deeper, better defined bass, more visceral sound.

Couldn't be happier.

I am happy with the sound....enjoy listening to music...have an incredible fleshed out sound with delightful micro dynamics that make the music accessible. So why would I need a subwoofer? 

Not “need”, but subwoofers would improve bass extension and support upper frequencies.

Bass reminds me of my drinking days: "I had three beers and felt good, why did I think that three more would make me feel better" 

My drinking days are over - on sobriety.  I still yearn for craft beers, Bass Ale was a favorite.

It should boost my testosterone as well. 

Manly masculine bass

I added an SVS SB-5000 to my JBL L100 MKII and it’s really taken the sound stage and music to another level with better fidelity, greater tonality. I was just going for more music below 40Hz so the grander sound improvements really were pleasantly surprising. 

I run balanced from the linestage out to the sub and cutoff the LPF on the SVS at 53 Hz, which is aligned with the -3db where the JBLs start to drop off in Erin’s speaker measurement review. Seems to have really integrated very well.

@soix 

Great suggestion. I’ve suggested listening to live acoustic bass, cello in other threads: it makes it easier to assess a component, whether speaker or electronics.

Stereo subs are the way to go. I had one sub at one point, a Nola Thunderbolt II, but then moved down around DC and saw a pair of the Nola Thunderbolt III, and bought them.

It’s not just better because now there are two, it’s better because when the right and left channel are not summing up two channels into one sub, you hear more separation of instruments (if that's how they were recorded).

It’s  unfortunate (and  maybe expensive) to do this, but once you hear stereo subs,  on something like Thus Spake Zarathustra (the RCA recording was very good), Or The Tom Tom Club's Tina Weymouth's bass,  your eyes pop. As good as it was with one, now there’s twice as much bass power - and dynamics. And dynamics bring a system to Life.