A 72-year-old man presents complaining of gradually worsening hearing loss in his left ear over the past year, accompanied by persistent tinnitus ('ringing') in the same ear. He denies any ear pain, discharge, or significant vertigo. Otoscopy reveals clear ear canals and normal-appearing tympanic membranes bilaterally. Weber test lateralises to the right ear, and Rinne test is positive bilaterally (AC>BC). What is the most appropriate next investigation to rule out significant underlying pathology?