>>1945739With the proper amp and listening to the right music on the right headphone I have not had any problems with fatigue. I can say that if you have a big head the k701s might fit a little tight. Best fix for that, outside of stretching them, is to put the k601 headband on them. It gets rid of the bumps that are on the 701 headband and is just an all around more comfortable band. I just put that in mine and I'm loving it.
Sound wise though, no fatigue.
The DT 770s are probably the most comfortable headphone I have ever worn. With a cheap amp on these or if you mess with EQ at all and raise the highs, you might get some screechy highs, and that will cause some listening fatigue. I haven't had any problems with them though and I drive them with an E17 Alpen when I'm out and about (like right now, listening to anticipate by screech, DnB).
