Hi Matt,
Our study, “The Spectral Extent of Phasic Suppression of Loudness and Distortion‑Product Otoacoustic Emissions by Infrasound and Low‑Frequency Tones” (https://link.springer.com/article/10.1007/s10162-021-00830-2), might explain your problem. We show that the suppressive effect towards the high-frequency basal end of the cochlear is much shallower (2-3 dB/octave) than predicted from the longitudinal basilar membrane displacement pattern. We believe that the strong electrical potentials, generated by the apical OHCs in response to infrasound (Salt et al., 2013, https://doi.org/10.1121/1.4789005), are underlying the suppression. These fields have a low spatial decay in the highly conductive fluids and have the potential modulate loudness up to fairly high frequencies.
Unfortunately, I cannot think of practical ways to solve your problem either.
Best regards,
Torsten.