TromboneChat

Is there is something similar to Bitsch or Boutrys for bass trombone?

You could just read Bitsch or Boutry down a 4th: read as tenor clef and drop an octave.

I was going to add that no one need tell me I can read the Bitsch/Boutrys down an octave etc. Thanks to Bruce as always.

Aren’t they hard enough as written?

Also, Bitsch and Boutry both switch between tenor and bass clefs.

There is a published version of Bitsch for bass trombone, which I think keeps the same keys but extends the range downward. It's good - it keeps some of the high register as well.

Check out the bass trombone etudes by Otto Maenz. They are challenging in some of the same ways as Bitsch, but with a more, um, German style. I think there are two different volumes of them.