Back off topic... as many of the above posts highlight including Twerk's, this thing is a journey. I think jumping straight from something like Pro-900 to TH-900 may be too fast to appreciate the differences, but each to their own. It took me quite a few purchases and headphones to work out what signature I was after.
I went (approximately) over the last 5 or so years: Shure srh840 -> Beyer T70 -> Pro-900 -> HE-400 -> TH-600. Each and every one seemed like a fairly significant upgrade to me and I was hearing and appreciating new things in my music collection. If you go straight to the 'summit' you're going to miss some of that journey and also you'll still feel the need to 'upgrade' one day - to what, $5k 'phones? There's nothing wrong with lusting after an 'upgrade' but being a realist and putting a hold on it until you can afford it. Its good sometimes to appreciate what you have until you're ready.
To me the TH600 was a massive step up from the HE-400 in so many ways, and easily worth over 2x the cost. But at the same time I remember being blown away when I first heard the he400s and that planar sound. As I also was when I got my first good pair of studio monitor speakers, until I eventually realised they were far from flat and were being heavily coloured by the acoustics of my room (reflections) - which is why I love headphones so much, the only 'extra' colouration may come from the source.
Anyway, personally knowing that the TH900 are only 5-10% better (or different) than the TH600, they're not different enough for me to upgrade from 600 to 900 (ever). Not interested, period. If I were to go a different route one day it would be to wait for Fostex to bring out something new and even better, or jump to something like LCD-X or Stax SR009. But to me those are not headphones for listening while looking at a computer screen and doing other stuff. They are for closing your eyes and sitting there and doing nothing but listen. Something I don't do very often, and I doubt many of us here do much. So its a bit of a waste IMO.
Anyways, a great home set of 'phones and an enjoyable office set will do me fine.
I agree with a lot of your points.
I have owned the Pro 900 for over a year, have probably spent 100's of hours listening to them. When I first got them, I thought they were the best thing ever. It took me a long time to see weakness in the sound, mainly midrange. Also, I did own the HE-400 (twice!) and currently a D2000. I've tried many others if you look at my profile (M-100, DT990, Grado 325, Ultrasone 2900, 750, 780, etc).
I agree, it is somewhat about step by step increments so you can appreciate the improvements. I know that it took my ears a while to hear all the little details and extra resolution made possible by the high fidelity equipment that I own. Had I jumped from no headphone experience to a Fostex TH-900, I agree, I probably couldn't fully appreciate the advantages.
I think the TH-600 or Th-900 would be an upgrade that I can appreciate at this point in time. Sticking to the Sub-400 dollar headphones will not likely get me to that NEXT step in soundstage, resolution, separation, etc as MortalCoil mentioned. It will only get me different sound signatures and/or side steps.
In the end, what separates the TH-600 and TH-900 is like 500 dollars, which is significant, but not insane. I think it's worth going for the best you can get, provided you can (like you mentioned) afford it. The upgrade
is there in the drivers, 1 tesla vs 1.5 tesla, not to mention the beautiful red earcups. I listen to 90 percent EDM, so it makes sense to go for the best closed EDM can.
Lastly, I'd rather own one world class HP, than 4 great headphones. Others may feel differently. I feel this way because you can only listen to one headphone at a time. The rest you own just sit there on your desk when not being used. It's expensive to own extra inventory. I think people fail to realize this sometimes, but it's an important point when you're tight on funds. I'm still in college, 22 years old, only work (for now) during the summer, and I don't have extra money to throw around.
You mention waiting for Fostex to come out with something better before upgrading, but maybe that "better" (eventually for you) would be the Fostex TH-900. I'm not sure why you would rule out the possibility with a "not interested, period." You may find in a few months or a year that you will be interested.