Subwoofer amplifier

Subwoofer amplifiers reproduce very low frequencies, usually in mono, since hearing isn´t good at detecting position at very low frequencies anyway. If the upper range of the sub end is low, up to ca 200Hz, both channels are mixed and a mono subwoofer will perform satisfactory.
If higher frequencies are reproduced in the subwoofer, 2 subwoofers may be nedded to keep the stereo which otherwise becomes mono.
The ability in human hearing to position sound in stereo becomes less under ca 300Hz, which can be thought of when listen to old vinyle recordings, where the low range is mixed into mono without affecting the stereo, in order to reduce the track width for the needle and thus getting more time on the record.

If 2-channel subwoofer system is preferred, it can also be done using two separate subwoofer amplifiers, then only one input is used on each amplifier.
In an audio system with a subwoofer, a separate amplifier may be desired to avoid any overload that would affect the other frequencies. A subwoofer amplifier requires sufficient power because human hearing is not linear and a stronger volume is required at low frequencies in order to be heard equally.