TH900 !

yep.. so a great replacement 

The TH900 is a more bassy variant with better mids and treble but indeed too expensive !
Bass is impressive and better in quality than D5000.

Also the Fostex is better than Denons.

Can't comment on the TH600 but you could get some idea from Sonic Sense:
https://soundcloud.com/sonic-sense-pro-audio/sets/headphones-test

Whether the step-up is really worth $700.- is a question only you can answer.
To me it wouldn't be as there is a limit to what I am willing to spend.
Others (who own one) assure me it is.

It's not neutral but rather more fun and 'pleasant', more of a 'luxury bath' than a nice clean shower.
Personally I prefer realistic sound over 'lush' but admit sometimes 'lush' is briefly more appealing

Try it before you buy it, preferably for more than just a few days if possible.
Most like 'impressive' but get annoyed afterwards.

If you like the D5000 for it's 'luxury' you may like the TH900 even more.

It's not neutral but rather more fun and 'pleasant', more of a 'luxury bath' than a nice clean shower.
Personally I prefer realistic sound over 'lush' but admit sometimes 'lush' is briefly more appealing

Try it before you buy it, preferably for more than just a few days if possible.
Most like 'impressive' but get annoyed afterwards.

If you like the D5000 for it's 'luxury' you may like the TH900 even more.

These are very good points, especially audition before investing. If I was restricted to only one set of headphones then my preferences would be HD600 or HD800.

The D5000 make a very nice complimentary set in my can library. Visitors that enjoy the D5000 sound are disappointed when I mention they are unobtainum. Which is why this thread.

Maybe TH900 is the only alternative, albeit more expensive.

I have the TH900 at the office out of a CEntrance DACport and it is indeed a very fun headphone. I don't miss the HD800 that much since the TH900 has sufficient details for the genre I listen at work.

As for pricing, since I live in a place where the retailer seem to be nuts and determined to sell everything at a price very much higher than what I could find online, I finally got it at pricejapan and largely avoided import taxes due to a surprisingly low declared value by the buyer. Might not apply if you are in any country where the customs authority seemed determined to go through all packages to make sure every import is taxed.

If you live in the U.S, you might want to wait for Massdrop to have it again though it might not go through if there are not enough people committed to it.