I have a Boss GT-100 and love it. Iíve used it with headphones before and only had my iPhone in ear headphones the ones you get when you buy the phone (wired) and it sounded pretty good with them even with the output maxed.
Today I bought a pair of over the ear Sony headphones for about $200. Listening to music on them sounds amazing. However plugged into the GT-100 isnít that great. I had to turn down the output, because they were breaking up and for example the stereo pitchshifter effect I have on a couple patches doesnít even sound like it, where as on the cheap Apple in ear phones sounded awesome.

Any experience with this, or settings I should look into? It doesnít sound that clear, and I have the output on the GT-100 set to line out/phones as it should be.