Well, they dont know what efficiency (how much of the electrical power that is converted to sound and not to heat) i have on my headphones
-I am familiar with the fact that there may exist some kind of recommended or maybe even legal limit somewhere in the world, but i think it is ridiculous since efficiency easily can vary 20-30dB between different headphones)

However, currentlimiting on the line-out would solve that.(a simple resistor?)
line input usually is in the 10Kohm range for modern pro equipment and more like 100K for homeequipment (Really old tubegear can be as low as 600ohm but i dont think anyone ever will connect a cellphone to that ) while headphones are more like 8-32ohm and a few professional models with as high impedance as 600ohm.

Ps, about you PM, i am quite convinced that it is the dsp i cant get my ears around, so nothing strange there.