How do I know if I need a sub woofer?

I’ve got very good audiophile equipment and use bass traps and had help with setup - and to me the system sounds very good but not great!  I’ve hit a wall trying to make a less than ideal (family room) into to a great listening room. After attending Axpona the last few years, I am seeing some very high end Co add subs to their very expensive setups. I believe it’s for the same reason that I need them - poor room acoustics firming a less than ideal listening area. 
Like a few of the comments said, it’s less for deeper bass and more to affect standing waves that are limiting superior sound. 

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.