I'll do a bit of a comparison for you.
bass/
the th600 has a more elevated bass response compared to the alpha dog this is especioally noticeable in the sub bass which you can feel tickling your ears whereas I do not get such a sensation with the alphas. the sub bass is noticeably forceful, the alpha is not bad per say but not bass head level. the mid bass of the th600 seems slightly elevated as well, more than the alpha dog. the th600 bass is well controlled though even though it is elevated. the alpha dogs bass is very clean and is never bloated or overpowered to me, bass guitar is well in line with guitar and not muddy.
mids/
the alpha dog have a very detailed and complete mid range, however in comparison to the he500, lcd2 the mid range seems a bit thinner and slightly emphasized in the upper midrange. there is more detail in the mid range than on the th600. the th600 mid range is referred a bit and much thinner than planars. the kids seem a bit dryer/less smooth than the planars.
/treble
the alphas are a bit north of what I consider neutral, can sound slightly splashy on bad recordings or the wrong recordings. overall though I find them both detailed and mostly smooth. gorgeous treble. the th600 can be glass like at times and I consider the treble brittle at lower treble frequencies. most recordings seem OK but it is a problem at times.
soundstage and imaging/
I find these two caterrgories to be strengths of the th600. I feel that the th600 has a more sizeable soundstage with very 3d placement. I find them better than the he500 and lcd2 in this regard. the alpha dog soundstage is still very good for a closed can and is well as head of the m100, Phillips l1, momentum etc. imaging/placement is above the hd600 on my setup.
anywhoo I've run out of time tho post on my phone. excuse any mistakes/capitals.
as always YMMV, IMO ext