The simplest way to look at it is that, by connecting a sub to the output in addition to your regular speaker, you aren't asking the regular speaker to do any more than it would be doing without the sub connected.
However, because there is no high-pass filter on the main speakers, while the sub is giving you more bass, you also aren't asking your main speakers to do any LESS because the sub is connected.
With full bass management, you are getting more bass from the sub, AND reducing the amount of bass your main speakers are being asked to deliver... and so enabling them to work less hard.
In general, the purpose of a crossover or bass management is to route specific frequencies to specific speakers or drivers.
When you add a sub to a normal speaker, what you normally want to do is to route all the low bass to the sub, and all the rest of the frequency range to the speaker.
When you do this you are actually doing
FOUR distinct things:
1) you are routing the low frequencies to the sub
2) you are NOT routing the higher frequencies to the sub (you are blocking them from going there)
3) you are routing the remainder of the audible frequency range to the main speaker
4) you are NOT routing the low frequencies to your main speaker (you are blocking them from going there)
The reason why we want to route the low frequencies to the sub is sort of obvious - it's because the sub does a great job of delivering low frequencies.
The reason why we block high frequencies from going to the sub is twofold.
First, if we let the sub deliver high frequencies, which are pretty easy for us to localize, it will mess up the sound stage.
Second, most subs don't actually sound very good when trying to reproduce higher frequencies (because they aren't designed to).
However, because most subs, including ours, have their own crossover controls, we can simply send the entire signal to the sub, and use the controls on the sub to control this.
Now, as for the reason why we want to block the low frequencies from continuing to go to our main speaker.
The main reason is that our main speaker doesn't do a very good job of playing them.
Some speakers simply don't sound very good when attempting to play very low bass.
With others, when we ask them to make very low bass, it increases the distortion they generate at other frequencies.
And, with some, trying to reproduce very low bass may cause them to move dangerously far, or to use dangerous amounts of power.
(This is especially true for some bass reflex speakers when you ask them to play very low frequencies... they tend to move excessively far at frequencies below where the cabinet tuning controls the driver.)
And, finally, the amplifier that powers that speaker will end up wasting a lot of power amplifying those low frequencies - which the speaker won't play well anyway.
The real answer to that question is that "it depends on the speaker".
Some speakers, while they cannot play very low frequencies, "don't mind trying".... and it's perfectly safe to run them full range.
(In fact, due to the nature of crossover filters and rooms, doing it that way may make it easier to get a smooth transition between the speaker and the sub.)
HOWEVER, some speaker
DO mind being asked to reproduce those low frequencies...
They're using more amplifier power, getting warmer, waving around more, and giving you nothing in return.
The Airmotiv speakers were designed to be full range - so they don't especially mind being fed low notes they can't play.
However, by blocking those low frequencies they can't play from reaching them, you would enable them to play louder without distorting.
(they're not wasting energy trying to play low frequencies they are unable to, so they have more left over to dedicate to the frequencies they're good at.)
This is a bigger issue with movies than with music... because movie sound tracks often have really powerful low bass.
(And, because they really expect you to be using a sub for movies, they may include ridiculously loud super-low frequency effects that no normal speaker will enjoy trying to play.)
If you want to "do it right", you can add a passive line level crossover between the DAC and the speaker and sub....
This will both route the correct frequencies to the correct places, and block the very low frequencies from reaching the main speakers.
(But it will require more careful setup.)
Something like this (link) will do the job:
www.hlabs.com/products/crossovers/index_files/Page384.htmIn a friend of mine's set up we used Airmotiv 6's powered by an EgoDac and then added an Airmotiv S10 subwoofer with a Y splitter (one pair of outputs to the 6's and the other to the S10). I then set it up using the cross over controls on the S10 such that sub woofer blended in to the 6's natural low frequency curve. Noting the frequency response of the 6's I initially set the S10 low pass filter at around 70 hz and then adjusted till it sounded harmonious to us (to around 65 hz if I remember rightly). Later on I REW'd it and made very minor adjustments using the controls on the S10.
He is now using my USP-1 so he has bass management but still runs the 6's full range, we think it sounds better using their natural frequency roll off.
Cheers
Gary
Does this work with movie soundtracks with a heavy low bass content? I saw a you tube video where the reviwer's expensive book shelve sized speakers were damaged playing a movie without the low frequencies crossed over to a sub. I think I might be obsessing over a non issue.
