I like this list a lot, I can't disagree with anything on it as the only ones I might disagree with I haven't heard, which is why I'm not disagreeing. I know Oj has heard the PM-x2 and I think it's a valid upgrade from the HD600, I think the PM-x2 takes the tone of the HD600 and scales it up a bit. $1,100-$1,200 more though? That's where it gets hazy, but I do think it's more of a direct upgrade than the HD800 as I find the HD800 to be more on the analytical side while the HD600 sounds more musical.
Agreed on the Mid-fi purgatory, more or less, though.