Electric bass guitars are obviously a very important part of the sound and that's why a lot of care was put into their amplification. The norm was to connect the guitar to an amp, and then either the amp would send a direct signal to the P/A or a microphone would be placed in front of the amp and then the microphone would get linked to the P/A. A bass monitor was placed on Brian's side of the stage so Brian could hear what John was playing and they'd both be synchronised.
John was, for most of his professional career, a Fender Precision bassist. He bought a 1965 sunburst model by his second year at university (1970) as confirmed in one of the interviews (previously we thought he got one from Trident in 1972) and by late 1974 he had another one (most likely from EMI), which was made in 1968. The two basses are almost identical and they sound 99.99% alike. The only slight difference is on their logos: the 1965 model has a grey logo, the 1968 model has a black logo. John used both of them interchangeably for most of the 1970s, and then mostly the 1968 model from then on. Those basses underwent several changes on their finish: in summer 1975, John stripped off his 1965 model and left it natural, one year later he did the same with the 1968 model, and by the time the Magic tour began in June 1986 he had his 1968 bass refurbished and painted black.
From 1974? to early 1977 he took those two Precisions on the road, so whichever he chose for each particular concert would be his main and the other would be a spare in case anything happened such unforeseen damage or serious tuning problems due to an abrupt change of temperature, etc. He also had other Fender Precisions throughout the years: