A long tail pair can produce voltage gain. Then the negative feedback can reduce distortion more.
A long tail pair produces distortion if the input signal is more than a little but with voltage gain then the input level is less.

 

How to bias long tail pair, I have seen many tutorial. But seems nothing work well...
I know the biasing of voltage divider in C.E. ( common emitter)

 

Are you in a university electronics class?
Then why are you not taught about these things?

 

My branch is electrical and electronics so, these types of topic are very less taught in class ( not in course)