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2.1 is two stereo speakers for midrange and high frequencies and one sub-woofer for deep bass frequencies.
Humans can hear from 20Hz to 20kHz but most speakers, even sub-woofers do not produce 20Hz properly and many tweeters do not produce 20kHz properly.
For good fidelity the distortion of the amplifiers should be 0.1% or less at all operating power levels. Many amplifiers have their maximum power spec'd at 10% distortion which is when the amplifiers are clipping like crazy to make the power numbers higher.
The amplifier power should be rated as RMS continuous power from 20Hz to 20kHz at 0.1% distortion or less. Some amplifiers are rated for peak or maximum power (double the amount of real output power) or "music power" which is instantaneous power for only a moment before the power supply voltage sags which is a lie.
Noise from the amplifiers should be listened for since it usually is not spec'd. A cheap amplifier produces high frequency hiss and low frequency hum.