Disclaimer: This loan has been provided by my friend, vAsurada. This particular pair is SR-003 mkI, not mkII. Measurement has been carried out at 6 mm away from the reference plane with its headband removed.
Released as a Pro-bias version of SR-001 mkII in 1997, STAX SR-003 features small form factor compared to other full-sized headphone from the manufacturer for portable usage, and a 1.5 μm diaphragm, which is the same thickness as the diaphragm of current Lambda models.
And the question is, can this IEM perform as good?
CON: Fitting is quite tricky, due to the IEM's large housing.
ON SECOND THOUGHT #3: Surprisingly, the rear duct of SR-003's headband is a part of the IEM's acoustic damping system. By adjusting the volume of air behind the driver, the headband duct cuts off 4.5 kHz while boosting 6 kHz. Consequently, the frequency response becomes similar to that of SR-Lambda series.