To more or less echo what David said, if you aren't going to spend a couple thousand dollars on amps and DACs, I don't think you could do much better than the HE400i. Throw a decent midlevel amp/DAC combo at it, and it shines. Out of my m9XX, with no EQ applied, I very slightly prefer my HD800 to the HE400i, but it's not a huge difference. With EQ applied to the HD800, it starts to pull away, as it just has soundstage and resolution that the HE400i can't quite match no matter what you have in front of it. I think the HE400i stands toe to toe with the HD650, even if you put a world class system in front of the HD650, but that's probably partly a function of slightly preferring the sound signature of the HE400i to the HD650. With a fully equalized HD650 and a fully equalized HE400i (through sonarworks), I think the HE400i completely matches the HD650, even if you feed the HE400i an inferior amp to what you're feeding the HD650.
So, basically to sum this up: the HE400i is to me equal to anything else in "mid-fi" and is much more amp versatile as well. It may or may not not be your dream headphone, depending on whether or not its signature fits what you're after. Some will prefer the HD650's smooth signature, some will prefer the LCD2.2's more authoritative bass and smooth/rolled off treble, some will prefer the excitement of a Grado RS2e/GH1. But nothing in mid-fi world will beat the HE400i technically. Plus the HE400i has the advantage of not requiring "just the right amp" to get the most of it. It sounds more or less the same when being fed a Schiit Modi2Uber as a Ragnarok.
Now, yes, when you get to the next level, the HE400i does fall behind. It acquits itself well, and doesn't sound bad, but it's not going to match a system built around a HD800/HD800S, HE560, HE1000, LCD3/4, Focal Elear/Utopia, etc. But at that point we are talking $1000-2000 for an amp and $300-$3000 for a DAC (I only say $300 because my mind is still being blown by the value of the Modi Multibit) and maybe a couple hundred dollars for the right DSP software, if you're using it. That's before we even get to the cost of the headphone. Non-head-fi friends of mine can clearly hear the superiority of my Sonarworks -> GoodHertz CanOpener -> iDAC6/Modi Multibit -> ECP Torpedo III system in comparison to say the modi-multibit -> HiFiMan EF2C -> HE400i system. Do they think it's worth the money? No, of course not. But they do recognize it's better, while not being afflicted with our obsessive sound quality disease.