Marq,
Good call on The Kingston Trio. IMHO, they might be the clearest example of how this genre really morphed into the mainstream pop music of the '60s.
They were hugely popular, and I know that Martin Guitars considered them critical to the company's success and credits The Kingston Trio with helping fuel the guitar boom of the time. When John Stewart replaced Dave Guard in the mid/late '60's, their music started to morph toward more mainstream pop. Stewart later left the band and released one of my favorite records of the '70's "Gold".
Marty