At one point, I was using my FiiO E11 with the iAudio X5L which was very bulky and weighed over 300g. I used these together with my AKG K550s.
My X5L has now given up on both the line out and the headphone socket so it is no good any more. I now use the FiiO X3 and that sounds great without a portable amplifier added. The E11 makes a small improvement to the X3 with my headphones though.