And me.. I pretty much like open headphones opposed to closed. However, the ADs are some really good closed headphones. They even do mids better than the TH900 IMO of course. Bass - the TH900 gets the nod. However, the ADs are not far behind. The treble while both are pretty smooth the TH900s are brighter and the ADs are on the dark side but doesn't seem to be recessed at all. The Th900 are more lively up which I happen to like, even that becomes ones preference.
Price no option I'll take the TH900s
Price per performance I'll take the ADs all day long..
Never hearing the Th600 - the ADs may come even closer to them than the TH900s.
From memory I'll take the AD's over the D2K and D5K either way. The D7K puts up a better fight. Really have to have them side by side to decide..