I also played Soundings for bass quartet its a great piece If you can find 4 people who count. If not It will be a disaster. Some good modern solo composers are Frank Proto, Francois Rabbath, Mieczyslaw Weinberg(listen to joel Quarrington , David Anderson, and Heinz Holliger(Listen to Edison ruiz). Two great concerto pieces from the 20 century
are Nino rota, Divertimento Concertante(listen to Edicson),and Eduard Tubin concerto(Listen to Edicson). None of these are as good as Hindemith(Listen to Rinat ibragimov) . Heres a link to Edicsons channel below
http://www.youtube.com/user/doublebassVenezuela
Listen to Joel
https://www.****ekta.com/en/album/Garden-Scene.571.html
Listen to Rinat ibragimov
http://www.youtube.com/watch?v=tg8Ntyi_6H8