Seems an area that companies will cut a corner on, in my experience.
I've had about 4 interfaces in the last few years with the headphone output varying massively.
These are my view, all using beyerdynamic dt990pro

Echo audiofire: Nice and warm, not tiring on the ears, maybe coloured the bass a bit but I was happy to use for hours. Sorry that's FireWire btw.
Focusrite vrm box : Sounds ok but not enough bass, not very loud and didn't sound so good when turned up, vrm function is pretty useless.
Focusrite Saffire pro 14 : Louder but I found harsh and tiring, lacked bass. FireWire again but I imagine has the same amp as the USB.
Apogee Duet 2 : Without a doubt the winner, nice and warm + there Is definitely more detail, go a lot louder than all the above without distorting. Mac only but the point is its a more expensive card.

I think if you intend on using headphones a lot it's worth looking at high end cards or getting a good quality headphone amp.