An increase in the dynamic level of a brass instrument from forte to fortissimo generally brings about a change in timbre to a much brighter sound, often described as 'brassy'. This phenomenon is due to non-linear wave propagation along the bore of the instrument, which generates significant spectral enrichment in the higher harmonics. The dynamic level at which this change occurs is not always clearly defined and some brass players have found that, by employing slight changes in embouchure, they have a degree of control over the level of brassiness at a constant dynamic. Experimental data from playing tests are presented, showing that players employing this technique are able to modify the gradient of the input pressure wave front as it is formed in the mouthpiece. Numerical simulations based on weakly non-linear shock theory, with experimental data from brassy and non-brassy mouthpiece measurements as the input boundary conditions, confirm the assumption that the increase in spectral enrichment is primarily a consequence of enhancement of non-linear wave steepening resulting from a change in shape of the waveform in the mouthpiece.