How do I know if I need a sub woofer?

I added an SVS SB-5000 to my JBL L100 MKII and it’s really taken the sound stage and music to another level with better fidelity, greater tonality. I was just going for more music below 40Hz so the grander sound improvements really were pleasantly surprising. 

I run balanced from the linestage out to the sub and cutoff the LPF on the SVS at 53 Hz, which is aligned with the -3db where the JBLs start to drop off in Erin’s speaker measurement review. Seems to have really integrated very well.

@soix 

Great suggestion. I’ve suggested listening to live acoustic bass, cello in other threads: it makes it easier to assess a component, whether speaker or electronics.

Stereo subs are the way to go. I had one sub at one point, a Nola Thunderbolt II, but then moved down around DC and saw a pair of the Nola Thunderbolt III, and bought them.

It’s not just better because now there are two, it’s better because when the right and left channel are not summing up two channels into one sub, you hear more separation of instruments (if that's how they were recorded).

It’s  unfortunate (and  maybe expensive) to do this, but once you hear stereo subs,  on something like Thus Spake Zarathustra (the RCA recording was very good), Or The Tom Tom Club's Tina Weymouth's bass,  your eyes pop. As good as it was with one, now there’s twice as much bass power - and dynamics. And dynamics bring a system to Life. 

I just changed amps...when putting the system back together, I first listened with no subs, then added 1 and later the other...each made a significant improvement in overall sound, not just deep bass...main speakers are large floor standers rated -6db at 23hz...

This is a timely post, at least for me. Two days ago I figured I'd bite the bullet and pull one of the subs from my HT setup into my two channel system to see if I liked it.  I did, but it didn't seem as seamless as I'd like, and even though low frequency is supposed to be non-directional, I couldn't shake the feeling that it was.  I spun three records and was ready to "call it a day" and return the sub to it's dark home in the basement, but figured I'd try a second subwoofer, mainly because so many here and in other forums I trust recommend the practice.  Good grief! I was amazed and listened to one record after another until 3 a.m. rolled around and I didn't want to be useless at work so I hit the sack. I've spent the entire day champing at the bit to get home and listen some more - now I have to figure out how to sneak two more subs into the house (my back/knees won't take moving the HT subs back and forth)! Low frequency slam was definitely a hallmark of this experiment, but the soundstage became deeper and wider, and everything just "grooved" more, I don't know another way to explain it. 

@hifiguy42 

You have learned well, Grasshopper...