My favorite headphones for pure music enjoyment are the Denon AH-D7200, Grado SR60i, AKG K240DF and the AKG K712 Pro.

I have the Sennheiser HD 800, Meze Empyrean, Dan Clark Audio Expanse, HiFiMan Ananda and Grado GS3000e. While they are all good at something and do provide a good listening experience, they are either solely focused on analytical listening or are good at a very specific thing.

If you don't like the treble on the Fostex, then try the Denon headphones. They are made using the same Foster Denki base which is used by Fostex but they are not as treble intensive. Go with the Denon AH-D5200 as it's the closest to the TH900mk2 (slightly less V-shaped but still bassy). The AH-D7200 is more relaxed in the treble and has a slightly recessed ear-gain region making it very non-fatiguing. The AH-D9200 is like the TH900mk2 but without the bass-boost, so you will probably not like it.