Fostex TH-900

The TH-900 sounds extremely V-shaped in stock form.
Bass is fantastic ; well textured with amazing slam and great extension (but doesn’t extend as well around 15-30hz as a good planar). Issues start with the midrange ; it simply sounds too thin for a lot of music. Acceptable for some acoustic, ambient or EDM stuff, but not for the rest. The lower-midrange is too shy. 

The treble is very hot around 6-8 khz and I hear another peak at 10 khz. I find the midrange and treble pretty unpleasant for most tracks I listen to. This is very disappointing because the dynamics, resolution, soundstage (yes, they sound wide for a (semi)-closed design) are there. Though not as good as an HD800 (similarly priced on the used market). The newer Denon, the D7200, is a much warmer (but softer) sounding headphone.

Hopefully, a simple pad change improves a lot the tuning. Many users put TH-610 or TH-X00 pads on their TH-900, as it cancels the low-mids dip and reduces a lot the treble. I also did that (with Denon D5000 pads, similar to the TH610 ones).

With the Denon pads, the TH900 still sounds V-shaped, but much warmer and agreeable. Bass remains the same, midrange gains a lot of body and treble is no longer edgy – there is still some energy around 8-10 khz. Putting TH610 or THX00 pads might slightly bump up the midrange and smoothen the treble. Soundstage is unfortunately affected by the pad change, considering the smaller opening compared to stock pads. The TH900 still dominate most closed headphones regarding openness, with these pads.

The E-mu Teak or Fostex TH-X00, both of which can be found used around $250-400, are the best values in the Foster bio-cellulose family. Paying $600 for an used TH900 (mk1) gives you the Urushi cups, a metal stand, slightly better dynamics, cleaner bass response and better pad rolling potential.

Measurements & Resources

On top, TH900 with stock pads. Bottom, with TH610 pads.

TH610 pads fill the low-mids dip around 400-600hz, and reduce a lot the treble around 5-10khz. It seems there is some bass lost below 30hz, but unless you’re playing 20hz tone sweeps, it shouldn’t be audible in most musics.