The Wagner tuba comes in two sizes, tenor and bass, and both are transposing instruments. The tenor tuba's written range is from the C below middle C to the G a twelfth above middle C, and it sounds a major second lower than written. The bass tuba's written range is from the F a twelfth below middle C to the D a ninth above middle C, and it sounds a perfect fifth lower than written.
The sound of the Wagner tuba is mellower than that of the horn and sounds more distant, yet also more focused. Bruckner generally uses them for pensive melodic passages at piano to pianissimo dynamics. They can hold their own in a forte tutti, of course, but Bruckner generally gives them sustained tones rather than melodic motifs in such passages. In Bruckner's Eighth and Ninth Symphony, the four Wagner tubas are played by four players who alternate between playing horn and Wagner tuba.