Answers: Phase Switch:
A phase control is usually on a bass amp. It simply flips the output signal 180 degree out-of-phase, the same piece as reversing the wire on the speaker. The rationale is that low bass frequencies have especially long wavelengths and depending on the placement of the woofer and the distance to the listener, the sound might realize you off-phase. This is just an effortless way to compare the two. Just flip the switch (or enjoy someone do it) while you listen to a good bass track and walk out it where the bass sounds stronger.
Gain:
This controls the input signal to the amp and is designed to clash the head unit's output to the amp's input. If adjectives head unit used a standard output voltage, a gain control wouldn't be necessary.
LP and HP:
These stand for "low-pass" and "high-pass" and refer to the crossover filter. LP is used for bass speakers and filter out higher frequencies. HP is usually for mid/bass drivers and removes the lowest bass.
Crossover Frequency:
Used surrounded by conjunction with LP and HP, this sets the center frequency which determines where on earth the filtering begin. It is in Hertz (cyles per second).
Subsonic Filter:
Used for bass, this is in actual fact just a high-pass crossover set to a awfully low center frequency, something like 30 HZ or lower. It help reduce frequencies that aren't hear or produced by the woofers, but the amp would still try to reproduce them, leading to distortion and more grill. Even if frequencies can't be heard, their harmonics can still affect other frequencies.
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