Aurgolo, just what I said. Adding a high pass filter to the main speakers doubles the price of crossovers, and cables increasing the expense and complexity of adding subwoofers. So, to save money and make hook up easier manufacturers just delete the high pass filter. Even if your main speaker can not radiate 20 Hz that does not mean it is not going to try to do it and you can see your woofers wobbling around. These long excursions distort all the other frequencies the woofer is carrying. A high pass filter is necessary so that you can create a smooth crossover better integrating the subwoofer and eliminate low frequencies from the main speakers and their amplifier. This also dramatically increases you headroom overall. Giving you less distortion and more volume.
This is one issue those of us who have been using subwoofers for decades rail against. You add subwoofer to get lower bass and to clean up your main speakers. Both issues are just as important. It is true that some types of speakers benefit more than others. For speakers that have a woofer that crosses out at 150 Hz the benefit is much less than for a speaker that's woofer crosses out at 2000 Hz. You can get a MiniDSP for $250.00 that will give you an excellent 2 way crossover along with full EQ cababilities.
This is one issue those of us who have been using subwoofers for decades rail against. You add subwoofer to get lower bass and to clean up your main speakers. Both issues are just as important. It is true that some types of speakers benefit more than others. For speakers that have a woofer that crosses out at 150 Hz the benefit is much less than for a speaker that's woofer crosses out at 2000 Hz. You can get a MiniDSP for $250.00 that will give you an excellent 2 way crossover along with full EQ cababilities.