Some laptops have an internal subwoofer. The correct corner frequency for the crossover filter to be used with such subwoofer is, let's say, 700 Hz. Definitely not 120 Hz.
Let's see what happens if the user sets lfe-crossover-freq to 700, and then listens to (2.0) music and watches a film witha 5.1 soundtrack.
Music doesn't have the LFE channel, so PulseAudio will synthesize it (correctly, except for the overall gain, which is tracked as bug #95021). Frequencies below 700 Hz will be sent to the subwoofer, frequencies above that will be sent to the main channels. So all is good.
A film soundtrack does, however, have an LFE channel. But it is intended to be played on home cinema hardware, where the crossover frequency is 120 Hz. So, the original soundtrack has some content between 120 and 700 Hz in main channels. PulseAudio will leave it there, because it refuses to remix LFE if it is already there. Result: tinny sound, because the subwoofer is essentially unused.
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