I’ve been skimming a book on McCartney’s bass skills (
Paul McCartney – Bass Master by Tony Bacon & Gareth Morgan) and the
Sgt. Pepper’s section indicates that many of his bass lines on the album were attempts at creating a tuba-like, "oompah" tone on the bass. McCartney’s grandfather had played an E-flat bass in a local brass band, and that may have served as inspiration. During the
Pepper sessions at Abbey Road an employee was dispatched by McCartney to go “buy an E-flat brass bass” - and of course there is the cover photo with The Beatles dressed in bandstand clothes.