How do I know if I need a sub woofer?

The last few pictures in my virtual system demonstrate how multiple subwoofers not only extend the low end response but also reduce room modes and concomitant ringing. This is objective. There are subjective improvements in depth and soundstage as well. 

I listen at low volumes late at night---if I want a little more bass, I use the loudness on my integrated. I will say that I did have a REL sub and sold it since I didn't use most of the time. But now I miss it. 

How do I know if I need a sub woofer?

   Only you can answer that.  Do you feel like you're missing something?  Do you live in a space where you won't bother/intrude on neighbors?  I noticed a lot of responses on what equipment to buy, but that's not what you are asking.

Asked another way…. How do l know what l need?
 

In retail the worst thing is to have a buyer who doesn’t know what he/she wants.

If there is a dishonest seller, he will convince them of what they think they need.

If the retailer is honest, he may well have a head banging/nightmare experience.

The same thing can sadly apply in real life situations/relationships.

If you crank something like- Welcome to the Machine, and the bones in your ears don't vibrate, you need a subwoofer. Need is subjective so this may or not pertain to you.