My Q701's sound really cheap and pretty much worthless through everything I own except my Yamaha stereo amplifier, even when matching the volume, and I'm not saying that because it's big and shiny, or because I paid a lot for it, because well it's scratched up, missing a knob and it was free. Without the amp there is very little bass at all, and drum hits sound weak and dull, not even a bit snappy, there is no crispness to the sound. When I add in the amp it is fixed, that is not distortion doing that. If your amp doesn't make a difference then your source already has a decent amp, or you've been ripped off.
Maybe it's because the Yamaha's headphone is run from the speaker outputs(don't know the resistor values used)? My Pioneer reciever can drive them to uncomfortably loud, but it still doesn't help it sound any better than being driven directly from my sound card.