The high-pass filter, lets all frequencies 'higher' than the filter pass, and then steps off or reduces those lower at the filter frequency at a certain slope.

So in the article, he is using the filter to filter out below 25 Hz, leaving more headroom for the sub, now that it doesn't have to reproduce the lower freq. It is not a hard cutoff, but slopes off below 25Hz at 18 dB per octave.

-Dave

M80s VP150 QS8s EP500s
ravenmanor.com/cinema/