Hi iNx.13arney,
I have an HD590, which is 120ohms as well. This should roughly have the same characteristics as yours in terms of sound and power requirement. Based on my experience, the soundcard on my laptop can drive it more than adequetely. In this case, adding an inline amplifier will only add more distortion to the signal path. So if power is the issue, I suggest not using an amp.
Unless of course tube harmonics ang habol mo, then its a different story. Tube is the way to go.