On amps like the the Burson Soloist or the Violectric V200, the TH600 and TH900 are very close, and most people won't really feel the TH900's price is justified. Sure, there's a slightly wider/deeper soundstage and there's more weight to the bass impact on the TH900 (though I think the TH600 might have heavier sub-bass presence, possibly due to longer decay?), but it's not night and day. Scale up in the amp department and the differences gets clearer. It's kind of hard to say put into words, tbh. There's definitely a difference which justifies the price. I'm not sure if it's the wooden cups (fwiw, TAD-Labs also used Betula Grossa in one of their most critically acclaimed loudspeakers) or the higher Tesla value or whatever, but there's just more smoothness and weight overall.
Have you ever drawn a gradient on one of these photo-editor applications? On some monitors, you can sort of see the "steps" between the graduated areas (dithering or whatever it's called), while on other monitors the steps are less visible, or even totally seamless. That's sort of the difference between the TH600 and TH900 in terms of note decays, particularly in the low and high regions.
Another observation, like the Denon DX000 series, Fostex seems to have quitely improved the TH series (or at least the TH900) without telling anyone. It only became apparent to me when I tried a friend's TH900. His was one of the first batches (back when it was priced at $2K) that came with that wire stand. The trebles were a lot less smooth than those on my own TH900. The mids on mine also sounded less recessed than his. Same amp/Dac (Burson Conductor), same source, btw. No idea if it's some snag in the quality control or an intentional change by Fostex, but I suspect it's the latter. Not sure whether the TH600 production exhibits this or not.
