For transducers, depending upon the measurement method used, there should always be a target response to achieve neutrality, at least. A flat line isn't necessary that target. At the listening position, the target response I prefer is similar to the old B&K target ( http://www.bksv.com/doc/17-197.pdf ) see page 4. [I've seen HF'ers and a few people here incorrect refer to this as the Olive-Welti curve. The O-W curve is the target / compensation for their headphone measurement system. Sean Olive did not invent this target curve. The B&K target was formally presented to AES in 1974, and sound engineers were already aware and using similar type curve since the 1950s or earlier.] Flat will sound like shit at the listening position. This B&K target also somewhat works with the V2 measurements for headphones on this site.