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Recent research suggests that the ability to finely tune vocal-tract resonances during trombone playing may constitute an important aspect of performance expertise. Artificial player systems, designed to reproduce the behaviour of a real player, often neglect this component by not providing any control of upstream resonances. However, they offer great experimental platforms for quantitative studies on sound production mechanisms, allowing independent adjustment of certain control parameters. An active sound control method was designed to investigate different conditions of coupling between the artificial lips, the downstream air-column and the upstream cavity during sustained tones played by an artificial valve-trombone player system. Upstream input impedance at the fundamental frequency was controlled through real-time adjustment of the phase and amplitude ratio between the acoustic pressure generated on both sides of the lips. The phase difference between the upstream and downstream pressures was swept linearly over a range of 120 degrees while maintaining a constant upstream energy and fixed trombone fingering. Observations during this procedure included: 1. fundamental frequency variations within a range of 10 Hz in the neighbourhood of a downstream impedance peak; and 2. variation of the downstream pressure with a peak unrelated to the downstream impedance, suggesting optimal phase tuning with regard to the mechanical efficiency of the lip-valve system.