The Jordanaires were the first.  They became more and more in demand and popular at the time. Rumor has it that they raised their price (which might have been due to the increased demand) and Elvis was upset and decided not to use them anymore. Also be in mind that if they could demand the price (with the market) they were within their rights, but Elvis I think felt betrayed. Also, they were likely very busy and either didn't need Elvis or maybe even had trouble working him in even though he helped put them on the map. After the Jordanaires, Elvis used the gospel group The Imperials. His hero, Jake Hess had started the group, but his health made him have to quit before Elvis used them I believe. The Imperials also sang backup with Jimmy Dean and there came a time when both Jimmy and Elvis needed them and they couldn't do both. For whatever reason, they went with Jimmy and Elvis got the Stamps with his hero, J.D. Sumner (who wasn't going to sing with the rest of the Stamps until Elvis insisted) 
 So, both he and bass Richard Sterban (later and current bass with the Oak Ridge Boys) both sang bass. 
 Quartet-man