@traudio , take a deep breath and let it out. You are on the right path. The only error I see in your approach is that you have to include your main speakers in the mix. The issues with subwoofer integration are the right two way crossover point and slope, phase and time matching with the main speakers and finally what most people call room control which is really speaker control.
For most systems the right TWO WAY crossover point is going to be between 80 and 100 Hz. This is NOT just a low pass filter on the subwoofer but a corresponding high pass filter on the main speakers. Trying to integrate subwoofers with just a low pass filter is ....fill in the blank. Relieving the main speakers of having to make low bass lowers distortion through the lower midrange and increases headroom. Through the crossover region the subwoofer has to be in phase with the main speakers and the sound from the main speakers has to reach your ears at exactly the same time as that from the subwoofers. If this is not the case you wind up with the sonic equivalent of mud. Phase is cyclic so you can be in phase but out of time. You have to be both in phase and time for the best performance. Room control is equalization to fill in room nodes evening out the frequency response at the listening position. The only way all three issues can be fulfilled is via digital bass management. The best way to accomplish this is with a digital processing preamplifier. These range from the very respectical MiniDSP SHD at $1500 with the best microphone to the $12,000 Trinnov Amethyst. My personal favorite is the DEQX Pre 8 which is going to be released shortly, price yet to be determined. There is also the Anthem STR at $4500.
Two subwoofers are mandatory. Do not even bother with one. Without two subwoofers and digital bass management with room control most people will be better of without subwoofers.