These filters are almost always on receivers built when vinyl was still king. They are used to eliminate extremely low frequency bass notes caused by warped records, feedback (i.e. sound from the speakers, going back through the turntable's base then back into the cartridge--bad!!) or if you live in a place with weak floors, helps to filter out footsteps. The subsonic filter on my 1983 Pioneer SX-6 is centered at 15hz & works quite well. IMO, the feedback was the most potentially damaging effect--the woofers in my speakers would flap around wildly, emitting this really scary sound. If you let that go on too long, it could damage the woofer. So watch those volume levels!
And reproducing subsonic frequencies also takes a lot of power, so eliminating them helps to restore the amp's power reserves. I always kept my Pioneer's filter turned on, but unfortunately my Technics SA-DA8 receiver doesn't have one, so I have to be careful nowadays playing LPs.
That's the first I've ever heard of subsonic filters in receivers, very interesting. Now a days they're used generally only in subwoofers that's why I thought this thread was in the wrong forum. I have Velodyne HGS-10 and it has a subsonic filter at 20Hz.
Lee a subsonic filter is not opposite of a crossover it's opposite of a low pass filter since it is a high pass filter. It allows the high frequencies to pass and stop the low, while a low pass does exactly the opposite. A crossover is both a low pass and high pass filter and redirects the signal it the appropriate speaker. For example a crossover set at 2000Hz will rolloff the lower frequencies and send some of them and the highs to the tweeter and it will also rolloff the some of the frequencies above 2000Hz plus the lows and send them to the woofer. A crossover isn't a brickwall in which every single frequency above 2000Hz goes only to the tweeter and 2001Hz will never go to the woofer; the sound is rolled off in each direction to make a much more natural sound.