As the name implies, Sub-woofers (i.e. below woofers) are meant for sub bass sound not bass. Too many times I hear sound guys and stereo buffs use subs to boost the bass. All that gives you is loud bass. What's the difference? Well bass is what you hear as tones. the bass guitar, the deep vocal, the low keys, the baritone sax. Sub bass is just that, Below bass. It's the wave you feel vs the waves you hear. It's the in-audible yet powerful silence that makes it all more of an experience of feeling and not just hearing. And if you don't think silence is powerful, try getting everyones attention at a concert by bangning on a drum, then try doing it by shutting off the sound. the latter will suddenly turn every head in the room in a instance. A 50,ooo watt sound system that can achieve silence between notes gains everyones fear and respect.
Many of you know why we divide up the lows from the highs in our woofers and tweeters. Even many of us create a special mids sections for our squakers. it's because in order for the woofers to produce those big 100 hz bass tones, it has to make big slow movements back and forth. If it is also going to be required (at the same time) to make the 1000 hz tones, it needs to pause and retract a little bit 500 times on it's way out, and another 500 times on it's way back. Next time you look at a waveform on your computer, notice the long up and down cycle above and below the center line. that represents the lowest frequency, and all the rest of the jagged ridges of that line are the to-and-fro movements of the speaker while it's making that big long one. Every time the line goes up the speaker is moving out, and each time the line turns down the speaker pulls back. Even though it's continuing eventual journey to the peak of the biggest wave, it's pausing and quivering, partially returning, sometimes even mostly returning and back out again trying to make all of those smaller waves, along with it's required biggest one.
Considering that, it may be obvious that the further apart the high and low end of that spectrum gets, the harder -if not impossible- it gets for the speaker to reproduce the waves. So we split it up.
Which brings me back to the Bass. in order to make those really big cycles and waves sound smooth, we need to take some of the ridges out. Bass at 100hz and 200hz can give you nice clarity to your bass, like tweeters give clarity to the highs. When you get in the 60hz range (and this is where I differ from some audio guys) your getting deep bass, but still tonal bass. It's the 40's and 50's that put the body shaking gut under the bass. Those frequencies are long excursions for a speaker to make, and if they can be made smoothly they can be the hugely powerful silence that gives every one pause and attention. Try setting your crossover settings at their lowest first and slowly bring them up till you begin to hear the bass begin to lossen up, then back it down again till you have tight and deep thunder that's not loud but jurasic park big.