I never heard him live. Wish I'd had. A baritone colleague tells me that no one- simply no one- had a more beautiful voice. And that's the point of bel canto, isn't it, singing beautifully? Merrill would have been 90 this week.
Merrill studied with Samuel Margolis, who also taught Jerome Hines, the great American bass. And who did Margolis study with? While that's been harder to ascertain, and despite the lack of sources, I do remember Margaret Harshaw remarking that he was "a good teacher," a thumbs-up from one of the 20th century's greatest voice teachers.
As for the particulars regarding Margolis' technique, Hines gave a fascinating interview that appeared in the NATS Journal which you can read here. In it, Hines gives the reader a clear picture of Margolis' technique, from singing in the 'dome' to keeping the voice young by singing fast scales with a full voice. It's the kind of technique that has a lot of gusto factor in it and is quite simple really, that is, if you can wrap your ears around it. You just do it. And Merrill certainly did if the Youtube clip below is any indication.
What a great voice!