I havent listened to the ultrasones or the xfades, so I cant tell you about those. But as far ass the 770's go... I love em. They definitely have an above average soundstage to them, meaning spacial feel to the sound will be accurate. Their bass is amazing as Im sure youve heard, and I am a self admittted basshead. At the same time the highs and mids are crisp and sound good. Naturally with such a booming bass, the highs and mids can be drowned out a little bit.
For the fiio amp Is say the e9i