How do I know if I need a sub woofer?

The only way to find out is to try one and see if you like it! What speakers are you using?

to hear and FEEL pipe organs as they were meant to be heard and FELT, you need to be able to reproduce cleanly down to about 16 cycles per second. precious few stereo speakers can do that trick. a pair of subs is mandatory for all us pipe organ lovers.

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.