A class-B output has severe crossover distortion. Audio amps use class-AB which has no crossover distortion.
An audio power amplifier has a certain amount of voltage gain. You must tell us the sensitivity of its input for 30W output into 4 ohms or 8 ohms.
You also must tell us the input voltage from your source which determines how much gain the preamp needs. It might have a very low level microphone or a very high level CD player as its input so the required gain is very different for them.