1. An audio playback device, comprising: a stream processor which divides an audio stream into a high frequency band and a low frequency band, separately signal-processes the audio stream in the high frequency band and the low frequency band to produce a signal-processed-high-frequency audio stream using a first processing method and a separate signal-processed-low-frequency audio stream using a second processing method other than the first method, and integrates the signal-processed high and low frequency audio streams into an integrated signal-processed audio stream; and a playback unit which converts the integrated signal-processed audio stream output from the stream processor into an analog audio signal, and amplifies and outputs the analog audio signal.