Here is a quote from 1979...
"On the amplification front, he is at present in the middle of a bit of an experiment. He basically uses Acoustic amp tops, but recently introduced some Sunn equipment. He used to use the Acoustic bins with flex speaker but found the sound a bit woolly. Now he is trying some of their 2 by 15 bins, two of which are on his side and a third is on Brian May's side. He also has one of the Acoustic amps driving a set up of a couple of 4 by 12s just for the top end of the bass. A couple of graphics is the only luxury he will allow himself in the effects field. " link