The Wall of Sound

The Wall of Sound was an enormous public address system designed specifically for the Grateful Dead's live performances by audio engineer Owsley "Bear" Stanley. Used in 1974, the Wall of Sound fulfilled the band's desire for a distortion-free sound system that could also serve as its own monitoring system. The Wall of Sound was the largest concert sound system built at that time.

Source: http://en.wikipedia.org/wiki/Wall_of_Sound_(Grateful_Dead)

Several setups have been reported for The Wall of Sound:

1: 89 300-watt solid-state and three 350-watt vacuum tube amplifiers generating a total of 26,400 watts of audio power. 604 speakers total.

2: 586 JBL speakers and 54 Electrovoice tweeters powered by 48 McIntosh 2300 Amps (48 X 600 = 28,800 Watts of continuous (RMS) power).

This system projected high quality playback at six hundred feet with an acceptable sound projected for a quarter mile, at which point wind interference degraded it.

Source: http://en.wikipedia.org/wiki/Wall_of_Sound_(Grateful_Dead)

Vocals, lead guitar, rhythm guitar, and piano each had their own channel and set of speakers. Phil Lesh's bass was piped through a quadraphonic encoder that sent signals from each of the four strings to a separate channel and set of speakers for each string. Another channel amplified the bass drum, and two more channels carried the snares, tom-toms, and cymbals. Because each speaker carried just one instrument or vocalist, the sound was exceptionally clear and free of intermodulation distortion.

http://en.wikipedia.org/wiki/Wall_of_Sound_(Grateful_Dead)