For a beginner setup, I can't recommend the Magni+Modi enough. The Magni will have more than sufficient power to drive the HE-400, as that is what I paired with them when I first got them. Those headphones aren't picky in terms of amping, but going from a portable device, to a portable amp like the Fiio E11, to the Magni showed overall improvement each move up due to the increase in output power. It's still pretty amazing that the HE-400 can be driven off almost anything, though. The Magni could also be used to drive the HE-500 sufficiently, but is has the minimum power requirements if you ask me - something with 2+ watts of output power will be more ideal there.
From my personal experience, I think the HD600 is a really fantastic headphone with one exception - I believe they need a really fantastic amp driving them to be at their best. They aren't super picky or super dependent on an amp like the HD800 is, but pair them with a good OTL tube amp and they will sing. Going one further, pair them with a proper tube amp and they will soar. They're not going to be properly driven just out of a soundcard or DAC so something like the Magni would do the trick, though that would be sub-optimal if you ask me. Schiit's Valhalla or Asgard 2 would be a better pairing in that regard, so perhaps you'd want to consider either of these setups if you are considering an HD600 (but they may be beyond your budget): HD600, Schiit Valhalla (or Asgard 2) + Modi, -or- HD600, Bottlehead Crack and Schiit Modi (the Crack is a kit amp that needs to be assembled, if you think you'd be up to that).
I would put the HD600 properly driven right up there with the HD-500, but they do things a little differently so I'd still choose the HD-500 personally. When I first heard both, I got to hear them on a variety of amps to test (starting with no amp, SS portable amp, OTL headphone amp, speaker tube amp with headphone stage) and wow, was I impressed and really underestimating the HD600 the whole time previously. I can see why it is a favorite around Head-Fi and why people build systems around them. I felt the HD600, even properly driven, are a bit smoother, more relaxed and even a bit veiled in the treble when comparing them directly to the HE-500. I'd still regard both as ever-so-slightly warm but almost entirely neutral. On it's own though, the HD600 is one of the best headphones I've heard to date, and still a great listen with every genre I tried.
Also, consider that the HE-400 has a very unique signature (dark/warm but with sparkly highs, and fairly neutral frequency response with a dip in the upper mids) whereas the HD600 and HE-500 are going to be much closer to neutral, so no real emphasis on any one aspect. The HE-400 is a really enjoyable phone in some genres (like some jazz and all electronic styles), but it may disappoint in others. This is what I found from personal experience, and why I moved to the HE-500 - I can tell you that because of the harsh treble at times, the HE-400 with Miles Davis may be a bit intense and difficult to listen to. We have similar tastes in music, and I think you'd probably find something more neutral to be a better all-arounder when moving genre to genre. If you can swing it, the HE-500 will do just that effortlessly and flawlessly. Of course I don't mean to discredit the HE-400 any, because at it's price it's one of the best values out there, and I wish I could have kept it for those times where it really shined. I'd still consider it on it's own (like the HD600) to be one of the best values, and it will be the most cost-friendly setup that will destroy anything else in it's price range.
I know this probably doesn't make choosing your setup any easier, but I hope this helps some...I was in your same position once. My advice is to just continue researching all your options and read as much as possible if you are unable to test everything. Also, check out Dave's 'Flagship' review thread for a well detailed analysis of each: http://www.head-fi.org/t/634201/battle-of-the-flagships-58-headphones-compared-update-audeze-lcd-2-revision-2-6-4-13 I'd say his results are a pretty accurate representation of my experience with each.
Edited by modulor - 7/21/13 at 7:20am