nop, rather marginally inefficient vs others HP as LCD2 for instance 101db. 7db difference means over 5x times more power needed to get the same SPL. ERA1/Alara have quite low impedance(20 vs 70ohm LCD2), hence, they can get enough power from low voltage source as smartphones are. Inefficient ERA1 driver probably using ferrite magnet instead of common today neodim one. I tend to close my eyes for low efficiency of headphones because it is about milliwatts of dissipated power anyway(that afect battery live), however, lower impedance more difficult to drive precisely. The same amp will always have lower THD at 70ohm load than 20 ohm.