It all comes down to the DAC inside each device and how they pair up against one another. The HD380 is not a power hungry headphone and its 54 Ohm nominal impedance as well as 110dB sensitivity make it quite ideal for portable devices. Perhaps you could be a tad more specific one what you mean by better out of the laptop vs the portable device? Perhaps the portable device has compressed files vs your laptops uncompressed? It could be that your portable players has some volume limiting turned on or even an EQ setting.
Please list what player you are using on each device.
To be honest the DAC on the iPod Classic is actually pretty good and there really should not be any huge improvement using a laptops headphone out over the iPod.