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.
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.