Slow reply....
Been away from this thread for awhile and now just catching up again.
Ay advice ... Look up something called the K10 from Noble Audio. Easy to drive directly from the X5 (also from any other portable source I've tried), much more portable than full sized cans aaaaand most importantly ... eargasm!
I received the Noble K10 Universals I won at RMAF CanJam yesterday & with everything I have had a chance to put through them they sound amazing.
As expected (given the number of drivers & price) the sound is worlds above my Westone W4r's. The differences in clarity, soundstage, & fullness of sound when playing from the X5 & my desktop system are astounding. I am hearing things in songs that I have never heard before. I would dare to compare them to many of the higher end cans I listened to at CanJam.
As monkeybone mentioned, the X5 drives them easily in low gain mode. I currently have the volume at 65 & they are plenty loud. I would imagine an external amp would yield some slight gains, but I think it would be hard pressed to beat the stock sound by a large margin.