I read a very detailed review of these on Amazon (4-star by Mark Midura) that claimed that one needs an amp for these earphones:
I've had the M-Duo for about two months now, using them on and off either with my iPod Classic or through my O2/ODAC stack. I have noticed no extreme signs of burn-in, though I highly recommend using an amp with these. Without an amp, directly from my iPod, the M-Duo sound lifeless and a bit muddy. The sound really tightens up and becomes lively.
This is the first comment like this that I've seen. Has anyone else experienced this type of difference with and without an amp?