How do I know if I need a sub woofer?

@atmasphere 

+1. Again.

@sounds_real_audio 

If you decided to get subwoofers, maybe consider Audio Kinesis The Swarm as multiple subwoofers very beneficial to smoothing out room modes.

I use the Swarm subs as well and can also recommend them. That despite the fact my main speakers are flat to 20Hz. That meant I only needed a pair of the Swarm subs to do the job. What I like about them is they are designed to sit directly against the wall to take advantage of the room boundary effect, which allows them to be compact while flat to 20Hz and out of the way.

As do I. Again, extension and a reduction in room modes. I think it is the ultimate approach.

@atmasphere wrote:

I use the Swarm subs as well and can also recommend them. That despite the fact my main speakers are flat to 20Hz. That meant I only needed a pair of the Swarm subs to do the job. What I like about them is they are designed to sit directly against the wall to take advantage of the room boundary effect, which allows them to be compact while flat to 20Hz and out of the way.

Which Classic Audio Loudspeaker model are you using - the T-1.5 Reference? If so then these main speakers of yours have what effectively is a built-in sub with an 18" down firing woofer per speaker, in addition to a front firing 15" ditto. They adhere to physics alright with size, high sensitivity and large displacement area, and so by adding the two spaced Swarm subs by Duke you have 4 bass sources with the predominant role of the dual Swarms to acoustically smooth out the response, much more so than pressurization anyway - a clever approach. Actually the Swarms don't need to extend to 20Hz flat to do that, but it certainly doesn't hurt either. 

What I'm saying is this: most don't come with a pair of main speakers that like yours have the kind of low end foundation they do, and so your speaker setup is hardly representative. Rather it's typically the inverse scenario where the subs need to do the pressurization and extension down low, but similarly to the approach by you they could the leave their main speakers running full-range (at least ~40Hz extension is needed) and space out the (ideally large) subs to make for a DBA and to have proper low end fill and smoothness.