Its the groove, not the singing that gets them. Still, not sure whats the deal.

Having fallen out with Jim Stewart at Wexler’s southern recording spot of choice, Stax Records, he turned to Hall to cut his records in the south.

The Muscle Shoals sound

The Muscle Shoals style fused hillbilly, blues, rock’n’roll, soul, country and gospel, to create a sound that cherry-picked the best features of each to forge something new. They close-mic’d the kick drum, and the FAME recordings pumped with heavy bass and drums. But the playing was light and loose, the songs melodic and full of stories. And through it all was deep passion and grit.

One of the first acts Wexler sent to Muscle Shoals was Wilson Pickett. “I couldn’t believe it,” Pickett told journalist Mark Jacobson. “I looked out the plane window, and there’s these people picking cotton. I said to myself, ‘I ain’t getting off this plane, take me back north.’ This big southern guy was at the airport [Rick Hall]… I said, ‘I don’t want to get off here, they still got black people picking cotton.’ The man looked at me and said, ‘F__k that. Come on Pickett, let’s go make some f__king hit records.’ I didn’t know Rick Hall was white.”

When Wexler came to FAME, he was shocked by the laidback nature of the sessions. He was used to working with the country’s finest session players, who would sight-read from charts, knocking out hits in a highly professional manner. But things were different in Muscle Shoals. Here the musicians were local guys who looked like they worked in a warehouse or supermarket. And yet, as he quickly realised, these were smooth and funky players, musicians who cut a groove to rival any in the land. Pickett and Wexler were bowled over and sold on the sound they had going on.

Wilson Pickett With The Muscle Shoals Rhythm Section. Photo courtesy of Alabama Tourism Department

It was while the others were out to lunch that Allman suggested to Pickett that he cut a cover of ‘Hey Jude’. Both Pickett and Hall thought Allman was crazy to want to cover The Beatles, but the finished record would be one of the greatest covers of any Beatles song, as well as one of Wilson Picket’s most powerful recordings (not to mention a huge hit). On hearing Allman’s playing on the record, Eric Clapton was knocked out: “I remember hearing Wilson Pickett’s ‘Hey Jude’ and just being astounded by the lead break at the end. I had to know who that was immediately – right now.”

Besides Aretha Franklin, Etta James, Rolling Stones' Sticky Fingers & Lynyrd Skynyrd, 

The boon the studio got from the Stones’ sessions can’t be underestimated. Muscle Shoals became the 70’ Funk Factory, while at the same time attracting the biggest names in pop and rock, from Paul Simon and Art Garfunkel to Rod Stewart to Elton John.

Feuds, ‘Freebird’ and The Fame Gang

The feud between Hall and Wexler meant that both studios had to up their game. Over at FAME, Hall put together a new band, dubbed The Fame Gang, and recorded hit records with Joe Tex, Tom Jones, The Osmonds, Candi Staton, Bobbie Gentry, King Curtis, Little Richard, Paul Anka, Bobby Womack and Clarence Carter. In 1973, Rick Hall was named producer of the year after records he’d made topped the Billboard pop charts for an extraordinary 17 weeks.

https://www.udiscovermusic.com/in-depth-features/muscle-shoals-studio-history/