[amp for x10]

I have used the Fiio E11 and it does a great job with my x11i's. Give just enough umph and the bass adjustments are great if you are listening to Mp3's or a streaming service (like rhapsody etc) that might not have optimum bitrates.

[X10 vs s4]

Seeing bwbike saying

After more than a year of using both, I still like the sound of the S4/S5 phones better

ands Thai saying

The S4 may lack some delicate detail, but it is more exciting to listen to

I think this may be due to the impedance of the s4 being 18ohms vs the X10 being 50 ohms. Phones and ipods are much more easily driven w/o a amp at that level, hence a stronger sound.