How to get more watts from a 1W amplifier?
"What is the "next stage output" used for?"
    "Can I put there more amplifiers if I want to gain more watts?"


Audio input signals (e.g. from a microphone) are usually very low power signals that must be amplified before they are useful with other circuits.  That's the purpose of a pre-amp circuit stage.  The next stage is usually for greater signal amplification, however, if not design properly, noise often gets added to the signal--which would degrade the quality of the signal.