Especially because the M500 has a higher than normal line-level at 6.8 resp. 3.4Vrms, enabling it to drive the L30 to clipping in high gain.
That being said, the M500s headphone amp can put out 126mW @150 ohms, wich means a more-than-healthy 116dB peak SPL with the HD 660S.
The noise performance is also fairly good and output impedance comfortably low - no need for another amp there.