Why The Watt is so big on Active Subwoofer?

[andymok]

because parts are big and heavy, air to move is also lots and heavy, and human are fairly insensitive to low frequency which needs a whole lot more to make up the perceived loudness

 

[DonH56]

Why do subs need more power? See e.g. https://en.wikipedia.org/wiki/Equal-loudness_contour and study this picture:

Look how much higher in dB low frequencies need to be with respect to middle frequencies to sound as loud. 10 dB more is 10 times the power, 20 dB is 100 times the power, and 30 dB is 1000 times the power. It takes much more power (all else equal) for very low frequencies to sound as loud as midrange frequencies.

 

[Chrispy]

A sub needs power. Most commercial versions do it via a built-in plate amp. Some use provided rack amps. Which is passive? If you must provide your own amp altogether that's more passive in my mind but an easy hurdle, depending on demands and efficiency and dsp requirements to extents.....need all the relevant information, tho.....

 

[Wes]

We should not beg the question of whether we want very low frequencies to sound as loud as midrange frequencies...

(my apologies to proboscideans and their buddies)

 

[Head_Unit]

Is not shaking like crazy but you can feel that the couch is at very least vibrating. I already put foam (3 inch) on three sub so they dont transfer the vibration to the floor. I suspect that was making my couch vibrating/shaking.

Be sure to put some foam on your head so it doesn't vibrate too much as well
Back to your original question, typical subwoofers these days are small compared to the speakers you are talking about using. The small air volume acts like a very tight spring. To bring the resonance down to a useful frequency for a subwoofer, the cone must then become heavy. This makes the whole mess inefficient, therefore a lot of power is needed compared to a big box with a sensitive driver.
- What are you using for a crossover, to filter the high frequencies out of your "subwoofers"?