The Guarneri Quartet was founded on the rubble of the Budapest Quartet in 1964. Famous for Bartók and Beethoven, the Guarneri not only had a very busy schedule of more than a hundred concerts per year, but also had a prolific recording career of 45 years. They stopped in 2009, and two of their members have since died.
Ruttencutter of the New York Times compared them to Leonard Bernstein because of what they did to popularise chamber music, mainly in America. Bernstein had his TV shows and brought attention to symphonic music. The Guarneri did that by giving interviews, being open about their métier and lots and lots of concerts. When the quartet started there were about a dozen American string quartets, and in 2009 more than 250. I highly doubt its all chalked up to their efforts though.
Although they’re named after one of the big three Italian string instrument builders in Early Music, only one of them actually did use an original Guarneri instrument, and just for a while.