> > Well that's the problem then! Most line outputs simply aren't intended
Try adding series resistors in your jig above. Start with 1K ohm on each of the channels.
Mess around adding different resistor values between the card and your headphones until it sounds better.
A higher value of series resistor will make the phones sound better, but you will have to boost the volume then. Optimum is earsplitting without clipping(you know the grind).