I would suggest a Mozart Horn concerto. The horn has a nice range, but not as good as ours (They just aren't as modest about it), so they have nice range and the melismas match to what a tuba can do, so it's a win win situation. Any of those 4 horn concertos he did are great and it would be a great piece to work on.
I thank yo all for the suggestion I have decided to go with the first movement of 4th Horn Concerto.
Good luck, but I think you're making a mistake. It's not going to sound good down an octave with the orchestra. Look at the passage at F and how it's going to be muddied with the bass line. Same problem 3 before M.
