BSS Audio FCS-966 Owner's Manual - Manual (Page 15)

6.3 HP Filter
Rotating this control clockwise progressively removes low frequency program
material, which is useful for controlling sub-sonic signals that otherwise could
cause overload problems. Judicious use of this control may allow more boost
to be applied with the main graphic section, as unwanted lower frequencies
are removed before the main signal is boosted. The HP filter may also be used
to restrict the bass signal applied to loudspeakers that are unable to handle low
frequencies, for example small drivers used in a bar or restaurant.
If the HP filter is not required, the control should be left fully counter-
clockwise.
6.4 LF Contour
This control applies a gentle, musical, low frequency boost and cut shelving
response. It works in addition to the main graphic and provides a convenient
way of changing the overall sound balance without disturbing a detailed room
or speaker response that has been set up on the main equalizer section. One
way to think of the LF contour control is as a means of augmenting the main
equalizer in a way that can be easily adjusted and reset.