“I love Dan’s falsetto and the imagination in his lyrics,” Diplo wrote in an email. He added that the band’s sound fit his image of Kingston in the ’60s: “with bad men and hot women and raging night life. I couldn’t believe it was a modern act that had this classic sound.”

Knowing their future likely would lie elsewhere, the band members kept from Mad Decent a cover of the R&B standard “I’d Rather Go Blind,” also produced by Mr. Axelrod, who often works with Daptone. Mr. Axelrod had used the Frightnrs’ rhythm section on various remix records for the label and passed the song to Mr. Roth, who had been looking for a rock steady band for years.

The quartet wrote all but two of the tracks on its debut, which accents rock steady’s love of American doo-wop and soul. The songs highlight Mr. Klein’s reaching vibrato, and the band was taken by surprise while recording the album last summer, when Mr. Klein began to lose his voice, as well as his strength. “The entire time he seemed out of it,” Mr. Axelrod later said. “To be honest, Dan can be a real complainer, so we all chucked it up to him pulling his [expletive] at the worst possible time. None of us had any idea what was really going on.”

“I was walking differently,” Mr. Klein said. “My back was hunched forward. I was walking up and down stairs very slowly.”

By last August, he quit his day job at a bakery and found himself falling down frequently. “I thought it was a chiropractic issue,” Mr. Klein said.

In September, he was walking with a cane, “which, at first, was ‘pimp,’” Chuck Patel joked.

But by October, Mr. Klein had to use a walker. He had become so thin, one doctor suspected he was anorexic. An EMG test, used to measure the health of muscles in the body, in November finally revealed the truth.