What are AV receiver and AV amplifier?

I turn down the woofer volume to medium low, can hear a little bit of bass, i think i will get used to it later.
This is a cheap subwoofer, i check that its spec is ok, frequency response 20Hz-165Hz, power output 135W, max 230W, 10" speaker.
It was suggested by people to set the crossover frq to 80Hz, and i found that my woofer has a range of crossover frq from 80~125Hz
Optical coaxial cable can improve how much the sound quality, right now i am using normal coaxial cable to link my DVD player to the amplifier.