How do I know if I need a sub woofer?

So let me ask you if the soundstage is already ’excellent,’ why do you need an additional subwoofer to artificially expand it. What you experienced is likely the reverberation added by the subs, going beyond what the actual music content presents.
 

@lanx0003  The sub is reproducing frequencies and information not previously presented by the speakers because they can’t.  That’s not artificial, it’s just uncovering more of the recorded information that’s already there.  If you think that’s artificial then you go ahead and believe that. 
 

What more could one reasonably want from subs?

Did you not read the review of the Wilson sub?  That’s what good subs properly set up can provide and perfectly coincides with what I’ve heard, and that’s in a system that likely makes yours look like a toy in comparison.  If you wanna just go on convincing yourself that your system is perfect and subs would provide no benefit and are artificially enhancing sound then live happily ever after in denial, but as I and many others here have heard for ourselves you’re not even aware of what you’re missing.  Ignorance is bliss.

@sounds_real_audio “l am happy with the sound”

No need for a sub. “(l)….Have an incredible fleshed out sound”

You said it yourself. “So why would l need a subwoofer?”

So in this case, there appears to be really no need to waste any more time offering further advice in this discussion.

Next question …. 

@soix  Ignorance is bliss.

Speak for yourself, Mr.  Stick with your crappy streamer and DAC, and keep convincing yourself that you managed to blend in subs seamlessly. Do you even own a $9,000 Wilson sub? I suggest you stop recommending outdated gears or things you’ve neither owned nor auditioned for a meaningful period of time.

@ronboco I was thinking the same thing with respect to room size.  My system is in a large room with concrete floors.  I have the floot space to locate a sub on the outside of each main (full range towers) .  It sounds great.

But, if my system were in a small room, I don't think I would want subs. My speakers would probably do fine without them.  I think subs would be difficult to integrate in a small room.  I'm just guessing.  

You just might be surprised at what you're missing, in any case won't know until you've tried. Most are agreed this more than about filling in missing freq. This as much about improved sound staging through the cues offered by those lower freq that are either missing in action altogether or rolled off in main speakers. I'm getting nearly flat freq response down to 33hz in my room at listening position, subs not doing a lot here, but the added low freq cues greatly improved sense of spaciousness, could never go back to no subs.

And yes, I agree it may be hard to provide a coherent presentation when adding subs, I tired for years and absolutely could not achieve the kind of coherence I was seeking. I was and am extremely sensitive to coherence to the point I had to quit three way speakers for two way and single drivers for nearly a decade. Going back to three ways via present Khorns, took me years of listening and mods to bring these up to my requirements. Adding a pair of REL's really completed the picture,  with careful setup I've been able to achieve wonderful coherence. 

By the way there is nothing 'artificial' in adding subs. Subs are simply reproducing freq info contained on recordings, attaining flatter freq response and hearing those cues which improve sound staging is simply exposing what's contained within any particular recording.