i am currently driving the Fidelio X2 with my iBasso D7. My audio chain is as below:
Laptop (JPLAY 6.1A) > Uptone Audio Regen (Amber) > LH Labs LPS > iBasso D7 > X2
This chain has a very spacious and balanced sound, with smooth treble extension, excellent midrange and punchy extended bass without overhang. Instruments and vocals are very well defined and coherent with a very very black background. Yes the D7 imparts a slightly warm tonality, but the Fidelio sounds just delicious now after 300+ hours of listening.
For me these two chains work nice:
Harman Kardon HD990 > Matrix M-Stage > Fidelio X2
Pro-Ject Debut Carbon Esprit > Pro-Ject Phono Box > Matrix M-Stage > Fidelio X2
Bass is nicely defined and tight, mids are open and treble well behaved. I am taking back what I said about the sound being "warped in cotton"