Just to settle the idea once and for all:
A bass trombone and a trombone (tenor trombone, to be precise) read exactly the same part. The bass trombone has the capability through attachments (triggers) to play lower notes than a standard tenor trombone (one without attachments that allow it to extend its range). A bass trombone has a larger bore which gives it a fuller sound, but does not change its range (both tenor and bass are non-transposing instruments built on a B-flat harmonic series. The triggers add tubing to start the harmonic series on lower pitches, most commonly on F.
It's not the same as a flute/bass flute, clarinet/bass clarinet, trumpet/bass trumpet situation. Those are an octave apart.