JMoreiraBoth the X5-2G and the K7XX have a reputation for being neutral (doesn't affect the sound). IMO that's what you are paying for when you buy a hi-fi MP3 player, but others disagree. The X5 has appropriate impedence and should put out enough power (it's around the maximum for this headphone).
I just got the ship notice on my X5, so we'll see. I'm not expecting any surprises, both of these are high-quality pieces of gear.
JMoreiraCan't for sure with the 2nd Gen, but I have the first version and while it can get loud enough on hi-gain, it's not exactly properly driven and sounds a bit thin. You'd probably need at least a dedicated amp like the Schiit Magni to really power it (maybe some portable amps will do, but can't vouch for any in particular.) In any case, this is is far from ideal for portable use so if that's what you want you'd better look elsewhere.
JMoreiraI use them often with the X3 2nd gen and find the result nice and neutral, with the gain high and the volume 85 -100 the equaliser flat, seems to drive them well.