The EP400 requires a lot more amp power to achieve the same output at 20 Hz compared to the EP500 and the added power has to start being added at a higher frequency. This means the EP500 will play louder lower than the EP400 even though the SPL achieved in the higher frequencies of each is not dramatically different. It is one of those things that falls in-between all the specs but matters in the real world. I think to achieve more clarity of the real world performance we should make the EP400 specs more conservative.
President & Chief Engineer