Before the sound comes out from the speakers. It just bunch of numbers and electric current pulse. To drive the speakers we need to amplify that signal. Different size of the speaker has different need of amplification. We call that power "Watt" There are different class of amplification developed over the years. You can see below show the different approaches. Each has its pros and cons. Most popular ones are Class AB and Class D. See the list below for the details:
Efficiency is most important for the amplifier. Its calculated by power consumption compare to the output sound power. Class D is the best amount all. You can see all class amplifier efficiency listed below.
Recent years the advanced Class D amplifier makes it a popular choice over others. There is one more type of the amplifier that is also popular in audiophile world.
Why Old Tube Amplifier?
Tube Amplifier is an old design use vacuum tube to amplifier the sound. Modern days we don't really need it any more because its low efficiency. However, many audiophile still buying and making them because of euphoric distortions they add to the music. Many people referred this harmony distortion as warmth that makes the music sound better. Today it is popular to use tube in the pre amps to add euphoric sound. Below is the analysis of the output signal from tube amp you can see the rich harmony it add to the original sound.
OPSIS II used latest Class D amplifier that has >90% efficiency and less then 1% distortion at high volume to make sure the reproduced audio quality. This also ensure the lowest battery power consumption.