I need legal reasearh

My husband performs a tribute to Barry White with his orchestra. We have full orchestrated charts for our musicians. We are being sued under the grounds that they say that we do not have license to Barry White's Orchestrations and it is an infringement of their copyright. We do have a license record and sell Barry White music. But they say we do not have permission to use orchestrated music and perform with out orchestra. They have no problem with performing with the band. I need to know if there is anything legal out there that says we need a license to perform orchestrated music.