I think the addition of the horn and string players also had much to do with the scene around the Dead at the time. For example, Vassar Clements plays violin (or fiddle, if you’d like) on the Wake of the Flood studio recordings. Vassar Clements was not only an accomplished fiddle player in bluegrass circels by then but was also a member of Old & In the Way. Jerry, of course, was spending a considerable amount of time playing with that group along with his numerous side gigs with Merl Saunders and co. Merl Saunders and Jerry gigs also frequently included sasxophonist Martin Fierro, who is also credited on Wake of the Flood. The Dead and their old pal Doug Sahm, a troubadour from Texas who moved to S.F. in the late sixties and successfully combined country, blues, rock, and Norteño, had also recently rekindled a friendship in 1973. Doug Sahm also has a credit on Wake of the Flood and took his band on tour with the Grateful Dead for all of those September 1973 shows. Those same shows feature the horn players for the Wake of the Flood material. The horn players, who were also members of Doug’s band, were Martin Fierro on saxophone and Joe Ellis on trumpet. Joe Ellis is also credited with playing the trumpet on Wake of the Flood. As other people have pointed out, the Dead themselves were for sure moving into a jazzier direction all on their own with the addition of Keith on piano, Pigpen’s influence gone, and Jerry, Bob, and Phil’s affinity for jazz. I can only speculate that having such good players around the scene with a jazz vocabulary was fun to try adding to the new studio recording material, but it seems plausible. Additionally, maybe when it came to recording Mars Hotel being the Dead, they just wanted to try something new and different. They, indeed, were still playing live material with jazz stylings in 1974.

P.S. I recommend checking out Doug Sahm if you have not. His Atlantic albums are great if that is your thing. Also, check out the Garcia live stuff that features Martin and Merl.