Hi OP:
So without a high pass filter before the main amp, the main amp produced the same voltage no matter what the sub does, and the speakers attempt the bass frequencies as best they can.
From a technical perspective, adding a high pass filter before an amp can really reduce wasted power... but it's rare that it even matters outside of professional applications. What really matters is reducing the distortion of the midwoofer.
It is often highly beneficial to high pass the main speakers and, if ported, to seal the ports. Use as high of a cut off frequency as you can (80 to 120Hz), not the lowest. The real sonic benefits of a sub + satellites comes when you can reduce the distortion of the mid-woofer.
Best,
Erik