The traditional way to connect a sound EQ unit was to take the output of a hifi receiver's Tape Out sockets and feed them into the EQ unit. Then connect the EQ unit's output to the Tape In sockets of the receiver. This meant that you could quickly check the effect of EQ by pushing the Tape Monitor button on the receiver.
In a more complex AV receiver there are many more inputs and outputs but the principle remains the same.
You need a line output from the receiver and a switchable line input to the receiver that doesn't interrupt the selected original source.
The EQ unit is connected in between.
I personally don't like the extra noise and distortion that most domestic EQ units introduce and would avoid them.