output impedance is not the sole criterion for determining an amp's bass drive and its synergy with headphones. perhaps this review (altho referencing a diff. headphone) might alter your perception http://www.head-fi.org/products/grace-design-m903-reference-headphone-amplifier-dac/reviews
but you shouldn't use head-fi reviews and commentary as your only guide. if you can, try to get to a headphone meet & try the amp in question ... you might be happy with the results; perhaps not - only you can judge.
Edited by geetarman49 - 8/23/12 at 2:34pm