The 70s were definitely the golden age for Marshall. Not only were Marshall amps sold worldwide, they were seen on stage with the best musicians. These musicians often reflected the heaviest music styles, and many of them happen to be British as well.
Aside from the usual suspects such as Pete Townshend, Eric Clapton, and Jimi Hendrix; The 70s saw (to name a few) Jimmy Page, Jeff Beck, Ritchie Blackmore, Marc Bolan, Gary Moore, Angus Young, Paul Kossof, Billy Gibbons, Ace Freshley, Joe Perry and Eddie Van Halen (this list could go on forever) all adopt Marshalls. Only the loudest amplifier could serve the loudest players.
The mid 70s also saw the development of a master volume amplifier (later to be called the JCM800). This allowed guitarists to achieve the distinct overdriven sound of Marshall without the full volume of a 100 watt amplifier. This amplifier would become a significant part of Marshall in the next decade.
Aside from Rock n Roll, the 70s also marked a time when Marshall diversified by producing amplification equipment for instruments such as bass, keyboard and organ; along with sound equipment such as PA systems, mixers, and sound desks. This diversification meant that Marshall equipment weren’t just seen on stage with Rock/Metal bands, but also others who desired the utmost quality – musicians such as Elton John.
The 70s truly was a good time for Marshall (with the exception of their ongoing deal with Rose-Morris).
For those interested in Jim Marshall and the company he created, I highly recommend you buy:
Maloof, R. (2004) “Jim Marshall: The Father Of Loud”, San Francisco: Backbeat Books.