Help with Score/Playback sounds

Tonight is the first night I've used musescore and I love it; however, I am trying to write a choral score, but can't stand the sounds of the "choral ahs" because it doesn't articulate pitch changes clearly and the timing sounds muddled to me. Is there anyway I can build a choral score and change the playback sounds to piano instead? I've tried building the scores individually from the beginning, but I can't figure out how to change the clef and if I pick a clef it only allows for one voicing and not stackable staves. Anyone have any suggestions as to how to fix this?

Thanks for your time and help in this matter!