I saw one subwoofer amplifier with 18db 50hz boost.
What type of circuit they must hv used?
I will like to make one in my present subwoofer preamplifer.
Typically such filters are active types using op amps.
Unless you know the characteristics of your room and subwoofer, a boost at 50 Hz may just end up making the subwoofer sound boomy. If you don't have one, the best way to characterize your subwoofer in the room is with a sound level meter and a frequency test source (generator or test cd).
For accurate matching of the sub to the room, an equalizer such as the Behringer DSP-1124P can be used which goes for about $100us. It's use is described at https://www.hometheatershack.com/forums/bfdguide/.
I have an equalizer circuit that has a 10db peak at 35Hz. Then my 8" woofers sound and vibrate things like a sub-woofer. They rolloff below 60Hz so the EQ makes a big difference.
I haven't heard mains hum for many years. Years ago even my magnetic phono didn't play mains hum.