How do I know if I need a sub woofer?

"I guess if I get a sub or two my quest for a very good sound system will be over...thank god."

@sounds_real_audio      HaHaHa... I hope your right and not just wandering in another tunnel in your Rabbit Hole.

BTW, I haven't gotten motivated enough to approach the sub placement or settings with an acoustic analyzer as I am very satisfied with just putting the subs where I want them in general and tweaking the crossover and input levels to MY TASTE.  

I think you should consider adding the subs, enjoy the improvement and recognize that doing the acoustic analysis and attempts at room correction as a separate tunnel for another day.

This whole debate is just like a runaway train.

l have never had a problem integrating a sub into my various set ups with multiple changes of amps and floor/stand mount speakers. Maybe by just good luck, or the hand of fate in buying the right models that by mere chance worked for me.

The first was a near bottom of the range Rel Quake in 2004 with a meagre 100watt power handling. The step up in quality and soundstage led to a second buy to make a stereo pair in 2006. They certainly punched anbove their weight. Onward to a Rel T7 200watt after one of the Quakes transformers failed after 15 years. Surprisingly the experiment with the second hand larger sub paid off and it blended in effortlessly.

Today l run two Rel T9i’s in the same sized room and l’m as happy as a lark. I know it’s all been Rel on this journey, but if all this was a pure chance success thing to go by…. Buy a Rel or two and always wire through Hi Level Input for two channel. I never went down the room treatments path as l was content with just correct placement. It’s been the right choice for me, and l’m never thinking of, do l need to upgrade again all the time.

One thing l have learnt with well integrated subs is how much lower volume l need to listen to music. Lots of people will say subs mean higher sound levels and annoying neighbours. Not true. Without subs many with inefficient speakers play music too loud to reproduce the base they are missing.

I am happy with the sound....enjoy listening to music...have an incredible fleshed out sound with delightful micro dynamics that make the music accessible. So why would I need a subwoofer? 

@sounds_real_audio Standing waves can cause bass notes to be cancelled in your room. No amount of DSP or room treatment can fix it since the amplifier power is being cancelled by the standing wave. 

But if you have a sub and especially if you have more than one, you can break up standing waves. Since bass in most rooms is entirely reverberant, you can use a mono signal for the sub if its below about 80Hz or so. This is because the 80Hz waveform is 14 feet long and by the time your ears have figured out its there, its bounced all over the room. 

Many recordings these days have bass well below 40Hz. You can spend a lot of money on speakers that are flat to 20Hz (like mine) and still not have proper bass at the listening position due to standing waves. Its a problem I had to deal with. 

The ear has a sort of tone control built in. If bass is lacking, even though the rest of the frequencies are flat, the system will sound tilted towards the highs. If there is too much bass the highs can sound muffled. So if you get the bottom end right, an immediately audible effect is the system will sound more relaxed and realistic.

That is why subs are so important. 

@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.