What do hpf and lpf switchs do
The abbreviations "hpf" and "lpf" stand for High Pass Filter and Low Pass Filter. They do exactly what their names imply. They allow frequencies above or below a specific setting, while blocking or attentunating frequencies below or above the setting. There's also a "band pass filter" which allows frequencies within a specific range below the high setting and above the low setting. Finally there's a "subsonic" filter which blocks inaudible frequencies below the threshold of hearing.
The hpf is generally used in speaker component systems to send the high frequencies to the tweeters. The lpf is normally used with subwoofer systems and is set to send frequencies below the setting to the subwoofer.
While most amps will amplify a wide range of frequencies, they are more efficient when the range is limited depending on their intended use. For example, a typical 2-channel amp might have a frequency response of 20-20KHz. When used to drive a set of normal speakers, you would not use either the lpf or the hpf, but simply let it send all frequencies to the speakers. But when used to drive a subwoofer, and operated in "bridged" mode, the amp can put all of it's amplifying power into frequencies than can be used by a subwoofer if the lpf switch is turned on.
Hope this helps.