Adding subwoofer to Full-Ragne Speakers question

No, it does not work really work that way.

I'm no EE, but I'm gotten different answers to this. There are a number of different factors at work.
1. The bass waveform places a significant demand on your amplifier. If you use a powered sub and cut off the low bass with some kind of crossover, then the main amp does not have the same demands placed on it and has more current reserve available.
2. Reproducing low bass can also place a significant demand on the main woofer. If it does not have to go as low, there is a good likelihood that there will be less distortion in the bandwidth it is producing.
3. Most (but not all) full range speakers cannot produce the bass in the lowest octave at the same volume. In other words, they roll-off the lowest frequency. Or another way to look at is "how low can they go". A good subwoofer can usually provide useful, undistorted output down to 20 hz; few "full range" speakers go that low.

I'm sure there are others with more technical know-how, but that's my understanding.

The folks at Audio Concepts agree with Swampwalker's #1. Since they sell powered sub-woofers, they designed high pass filters to help the amp out. These would be if your preamp or processor has no built-in crossover controls. These filters come in both SE and balanced. Since they have a 30 day return policy, the best way is to hear for yourself if they work or not.
http://www.audioc.com/accessories/Hipass.htm

i added a pair of 2wq subs to my 3a sigs and it really cleaned up the mid/upper bass. mids and highs also improved. the setup utilizes a 6db/octave highpass filter which is sampled then boosted by the sub. whether the improvement is due to less strain on the 3a's woofer or increased amplifier headroom, i'm not sure. probably both. but the systems performance did improve noticably.

aloha keith

Swampwalker gave a good answer. In addition to the amp issues he explained, I believe his #2 comments have to do with the main bass driver not trying to move as much air if the LF is cut off. Therefore, the excursion of the bass driver will be less and the inherent distortion will be less when that driver is trying to also reproduce lower mids and upper low frequencies (above your cut off). However, like most things audiophile, there are trade-offs, which in this case include very careful matching of the sub with your main speakers, meaning the actual type of sub speaker(e.g., size/ported/sealed/servo-type/power, etc.), as well as the phase, crossover level, and boost/volume. You have to be careful because many/most subs are actually designed for the loud impact of HT, and not particularly for music. Your success will also depend on where your LF driver is crossed over. If it acts mainly as a subwoofer already, e.g. crossed over under about 150 Hz, or has its own internal amp, then your benefit may only be the adding of the very low bass below what your main speakers can do. However, if your LF driver is crossed over high, up into the midbass range, then you start to get the other benefits discussed by swampwalker. Many folks with high quality full range or almost full range speakers (that go down into the mid 30's to 20's) believe their speakers sound better without the issues associated with trying to integrate a sub. Then there is the type of music you listen to, your room size and characteristics, WAF, and on, and on, and on....Good luck.