There are different paths you can go:

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improve your thx00 : lawton tune-up, new cups, new ear pads and dampenings...

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upgrade other devices : better amp/dac (saw an as big improvement getting a new dac than when i got my fostex), better cable and noise isolation...

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EQ or better EQ (playing with Q, harmonics, harman...)

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other over the ear headphones: to add to the aldready mentioned :Emu teak, denon ah9200, audeze LCD4-Z, jvc ha sz2000, Meze Audio Empyrean...

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going the iem way: campfire atlas, sony ier-z1r, 64 audio N8...

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going hybrid: i tried with my fostex over some iem : it's not that bad actually, you get mids, highs and isolation from the iem and the great bass from the fostex. Since the fostex is kind of mid recessed it is some kind of improvement.