Likely due to impedance? 400i are Impedance: 35 Ohms, and as a rule of thumb you want <1/8th the output impedance of your headphones for your amp (ex; headphone 80ohms, you'd want <=10ohm amp),[/edited, thx E1D1!] so; iphone: ??? Shanling Output impedance: 4.8Ω DAC Xonar STU Headphone out: 10 ohm Xonar D2X : 50 or 100 ohms from what I could find.. Now the funny thing is that with audio sometimes better isn't better. I think that if your headphone isn't well matched impedance-wise, it might affect the FR, and who knows it might happen that you find even what could be worse theoretically better sounding...! Ex; So who knows if ironically using a worse matched amp could result in a subjective improvement! Like it this case, if you have a somewhat bright-leaning headphone you'd prefer to be warmer, then a mismatched amp who's impedance isn't really optimal so affects the FR as above; adding some warmth, worse might just be subjectively better! But yeah, impedance is only part of the equation, might very well be that one is just plain better than the other and it offsets the impedance differences... But yeah amp matching is so important it might actually explain quite a bit of the so different comments for the same headphones!