I am still orchestrating my score for the musical theater. I have pretty much settled on my brass section: First Trumpet/Second Trumpet, doubling on Flugelhorn for a couple of numbers/ French Horn / Bass Trombone, doubling on Tenor trombone for several numbers.
Regarding that trombone... The score requires periodically giving the brass section that low bass punch (think Les Miserables or Sweeny Todd), which is why I need the Bass instrument. Most of the trombone part is in the middle range -- which, according to my reading, should be equally easy to play on the bass or tenor. But sometimes I need it to go above the staff. (Remember this is a small brass section that needs to sound bigger than it actually is and have a lot of variety.) Sometimes I need to simulate three trumpets. If one of the reeds is free, I use a soprano sax for the middle trumpet part. If the triads are lower, I use the trombone as the bottom voice (never going higher than F above middle C).
So, my question is this: do I really need to have the trombonist play two instruments? Everything I've read tells me that the notes above the staff should be as easy on a Bass trombone as a Tenor. The only difference appears to be the quality of the tonal color, which is not all that different, and the fact that Bass trombonists may not be used to playing that high. The question is an important one for theater pit orchestras because, I think, under the current musician's contract, they get paid more for doubling. So, it's generally better to have fewer instruments. But, of course, if it's going to sound bad (either because of the instrument or the difficulty of playing the part) I'll have to let him double.