The Fostex TH900 mods thread was born.
It applies perfectly to the TH600 as well. Perhaps with 3mm (instead of 2mm) damping in the cups.
You are right:
I've put an additional 20 hours or so on my Fostex TH600's and really enjoying them now. They are now winning more comparison's with my HiFiman HE-400's than they are losing. (This was not the case at first) Not sure if it's burn in, my ears getting used the Fostex or just I had inflated expectations for the TH600's. The bass seems to have tightened up, allowing more detail to come through. I'm definitely liking them a lot more than when I first got them.
I too went from the HE-400 to the Fostex TH600. I appreciate each for the sound and impact they each deliver. My decision now is to admit that I underpower them and determine what headphone amp is more appropriate without breaking the bank.
Try a Project Ember, I found it to pair well with both my TH-600 and HE-400. You can adjust the output resistance/impedance to match your headphones and you can do some tube rolling if you don't like the sound of the stock tube.