How do I know if I need a sub woofer?

Which Classic Audio Loudspeaker model are you using - the T-1.5 Reference?

@phusis (+1) I have a custom version of the T3, which is taller than the normal version to obtain the same box volume as the T1, which was too wide for my room. 

It employs dual 15" woofers (one is the TAD 1602; the other is a custom field coil powered woofer made by CAR). 

One of the Swarm subs sits to my left with its driver facing the wall (to insure its in the room boundary effect; moulding along the floor describes its distance from the wall). The other, also with the driver to the wall, is behind me and slightly to the right. Both are well out of the way of room traffic due to the small size and required placement.

Its very easy to demo how well they break up the standing wave in the room. At the listening position there's no bass impact without them on, although that bass is there along the room boundaries and elsewhere in the house. When I turn on the subs, the bass is the same anywhere in the room. Its not overpowering, its just there. I have them crossed over at about 50Hz to make sure they have no output above 80Hz, so as to avoid them attracting attention to themselves. 

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