The specs say headphone output: 10 Vrms, so P = V*V/R = 10*10/600 = 166 mW into 600 ohms. The specs say 150 mW into 600 ohms, so about 9.5 Vrms which is close enough.
The E9 can deliver about 7 Vrms into 600 ohms at 1% THD+N (audio-gd doesn't specify THD+N).
If the E9 is louder then the specs must be off quite a bit. Btw, do you happen to know the output level of the source you used with the E9? And you used the high gain setting I guess?