I personally think the X1 is picky with what you feed it, both from an amp and DAC perspective.
I've been doing a comparison with 3 $200 DAC/amp solutions for a couple of weeks now in the form of the FiiO E17/E09k, Schiit M&M and the Aune X1 Pro. Every one of them sound different to one another and the Fidelio X1 varies from all of them too.
The FiiO stack sounded the most veiled, laid back, and somehow... lack of dynamics with the X1, at least in comparison to the Schiit M&M and Aune X1 Pro. I don't like how the FiiO stack sounds with the X1, the warmish signature doesn't help with the borderline nearly bloated bass of the X1. Sure the E17 can be used to EQ the bass down and tighten it up a little bit, but then you lose that immersiveness that the X1 is so known for.
Schiit M&M on the other hand balances thing better, it gives more life into the mids which is nice. The soundstage widens a little more, the bass more controlled, tighter in response, more layered, better textured. I like how the M&M sounds with the X1, but there's still that veil, that laid back feeling on the X1.
The Aune X1 Pro then, completely gives me all I ever wanted in the X1 before... it becomes more aggresive, the bass tightens up so well and rarely ever (at least to my ears) bleed into the mids anymore. Immersiveness is still there, the mids became forward enough without being in your face. The soundstage while being ever so slightly narrower than the other 2 stacks, is IMO not an issue considering the extra details in the mids and even more energy going into the treble. It gives the X1 a lot of life, that laid-back feeling is toned down signficantly but yet nothing sounds fatiguing. It's the sort of dynamic punch you get from an AKG Annies, but with more bass.
I understand that the X1 was originally designed to be a beautiful, immersive bass driven open dynamic headphone with a laid-back signature to it. But because of this, the X1 cannot truly become a very well-rounded headphone and it's also a reason why there are half as many users wishing that the bass would be slightly toned down and being tighter.
The Aune X1 Pro is IMO, the best pairing with the Fidelio X1 I've heard in the $200 price range. Some may even say that I'm effectively just coloring the sound of the X1 (lol I would if there was hardware EQ controls, but there isn't), but whatever... it's not like the X1 sounds like an entirely different headphone. The X1 is still the X1 regardless of the pairing, many of its flavor remains, but it's just much more balanced with the Aune X1 Pro.
