Previously my front speakers need to handle low end bass also. So my front speaker not getting enough power from my HU (no external amplifier). So sounds more teruk lo.
Now my active subwoofer has its own amplifier built in, so it handles all the low end. My front speaker only handles high and mid. So my HU still can supply enough power to the front speakers, but still not enough for full volume. Full volume will have some clipping sound, if your ears sensitive you can hear it. Clipping sound means not enough power supplied from HU to speakers. But now the quality for the loudness I required is more than enough for me, so I'm satisfied.
This is what I understand from the installer la. 

the signal no longer send very low frequency stuff, so yeah clearer output for mid and high range, this doesn't matter much if you have good amp.
Bass unit no need to produce very low range frequency, so it could focus on producer higher bass frequency sound

for kenwood and jvc, you have this settings to filter out low range frequency, e.g if you choose 150hz, 0-150hz will not be send over front/rear speaker wire.


here a better settings explanation.
http://manual.kenwood.com/ce/im362/Asia_Au...M_en_00-14.html