I have been reading through the forum, here, quite a bit - but I find myself getting bogged down in the myriad contradictory opinions, so I thought I might outline what I'm doing and what I'm after.
I have just acquired a pair of HD800
Rationale being that people seem to often cite it as the headphone that caused them to be selling the headphones they were selling, as I was searching for my next pair.
I generally use headphones at night for music appreciation, but I also do production of electronic music during that time, so something that is relatively neutral is preferred. My monitors are quite neutral and I don't mind that. That's what I'm used to listening to.
I own Audio Technica AD900, AD900X and M30, already, with the primary reason for me buying the HD800 being that I have never, ever, heard a headphone that I thought was particularly great - and any better clarity, late at night will translate to better mixed stuff to be tested on monitors.
Here's the caveat - I'm comparing them to the feeling I get from sitting in the sweet spot of my monitors. Sitting in the sweet spot of a pair of Genelec 8030, with a 7070 sub (turned very much down). The sound stage of that combination (for me - and I'm sure the rabbit hole goes further) is phenomenal.
I hear people saying they prefer the sound of headphones to speakers - I don't get it. I put my money where my mouth is, though - I bought something that *might* challenge. Of course, the expense of that monitor setup is quite a lot more than a set of HD800 (not that I paid that - I think it ran me around 2.5k total on Ebay).
And my first impressions are that they don't sound much better (if at all) to my AD900X. Clearly a little more bass. But really, not particularly impressive. Not even very bright, as cited by most people.
But I'm only using the headphone out of my M-Audio profire 2626
So, I need to buy a headphone amp.
Of course,buying the HD800 cleaned me out, so I'm looking for something that can do a moderately decent job and keep me from thinking I've made a poor purchase - without making me spend too much money. I'm an "I've got to see/hear it before I believe it" kind of guy.
This situation reminds me a lot of when I couldn't believe that the required grind setting for coffee changes vastly with coffee type. I was proven wrong about that (lot of aborted shots where nothing came out) and I'm really, really hoping that it's true in this case, too.
I've heard tell of the O2, I've heard people say M-Stage, but I've also heard plenty of naysayers on those amps. In fact, I feel like with the right motivation, I could find someone hating anything on here, particularly with the cheaper stuf.
Budget is I would say 0-$500, no DAC required. I have more DACs in this machine than I want (also have an RME 9652).
I've been swapping between the 900X and HD800 whilst typing this. My opinion most definitely is not changing.