One solution to the resonance problem is to use a coupler with a smaller air volume. This pushes the coupler resonant frequency beyond 20 kHz, removing the artificial peak. The downside to using a small volume coupler is that the bass will be overstated compared to the ear’s response since the air volume of an ear canal is larger. In this case a 0.4cc coupler is used, available for example from G.R.A.S., combined with a custom interface.