For more than three decades I have strongly advocated the high-level connection of subwoofers—where we connect the output of the power amplifier to the input of the subwoofer.
What amazes me is that still to this day, that viewpoint is considered radical.
The vast majority of subwoofer manufacturers would have you connecting their subwoofers through low-level inputs as supplied by your preamplifier. Their reasoning is simple. The output of a preamplifier is cleaner and more direct than what happens after a power amplifier has processed it.
My good friend, John Hunter of REL subs is one of the few subwoofer manufacturers agreeing with me.
And here's the thing. The majority of subwoofer manufacturers are correct. There's no argument that the output of the preamplifier is cleaner, purer, and more direct than the output of a power amplifier.
So why the debate?
Because they are missing the point. Subwoofers should not stand out in the system. The whole point of a subwoofer is to augment the performance of the main loudspeakers. We don't want to hear the subwoofer. We want to pretend as if it were a perfect appendage to the main speakers. To make that happen we need to do whatever we can to get closer to matching the sound of the main speakers—a perfect pairing.
We want the characteristics of the power amp to color the output of our subwoofer in an effort to more closely integrate it.
Hope that helps.