The Wagner tuba is a relatively new addition to the family of brass instruments and could be said to contain physical and acoustical characteristics of both the horn and the tuba. It has survived the test of time, as it successfully provided the addition of a noticeably different tonal colour to the already rich spectrum of brass timbres. One of the most characteristic aspects of a brass instrument's timbre is how quickly the tone becomes brassy with increasing dynamic level. This paper describes an investigation into the brassiness potential of Wagner tubas and related instruments. Tests involved measuring the rate of spectral enrichment during a crescendo, using a loudspeaker positioned at the input of the instrument as the excitation mechanism. The relationship between the input and the radiated sound pressure at the bell of the instrument is explored. The results from these experiments are compared with values of the Brassiness Potential parameter derived from detailed measurements of the bore profiles of the instruments.