I have been doing an A/B comparison between the TH909 and the TH900, then I did A/B comparisons between the Fostex and the Sennheiser HD800S and the Audeze LCD3. Bottom-line is that IMO the TH909 is better than the TH900 and that it sits between the Sennheiser and the Audeze in terms of sound signature (a bit closer to the HD800S).
For background: I’ve used a Hugo2 and a TA-ZH1ES for this comparison. I am an addict for big soundstage and I like headphones which are able to retrieve excellent levels of detail, I’m not a bass head although I can appreciate good bass slam. I can equally enjoy my HD800s or my Z1R.
Compared to the TH900: They are both equally comfortable. Sound wise IMO the TH909 beats the TH900 in every category, except obviously in isolation and in bass quantity (not quality). The TH900 strikes harder hand sounds more bass-head in direct comparison, but bass feels also more bloated. TH909 has better bass texture, definition and extension.
The TH900 sound signature is more V shaped and highs are more piercing. The TH909 is a better all-rounder, highs are not fatiguing at all, micro detail is better, there’s more air between instruments and soundstage also feels bigger (not HD800S bigger).
I would recommend the TH909 over the TH900 unless you need a closed back and/or like more bass thunder. I certainly consider the TH909 to be an upgraded open back version of the TH900.
Compared to the HD800S: Despite the TH909 has very good soundstage, the Sennheiser is still (and clearly) the king in terms of soundstage. The HD800sS feels clearly a brighter sounding headphone and can make the TH909 seem warm sounding. Switching from the TH909 to the HD800S made me feel like making the music sound bigger, like going from an “L” to an “XL” size, it felt more in your face and immersive with the HD800S. For classical music, instrumental etc the Sennheiser is the better way to go, but if you feel need more “balls” (e.g. for Heavy Metal) then you may want to check-out the TH909; in direct comparison the HD800S seems to be lacking bass slam, the Fostex hits harder and has a warmer sound signature in comparison, while retaining a very good level of soundstage and resolution. I think both headphones can be complimentary.
Compared to the LCD 3: I got the opposite feeling than with the HD800S comparison. The Audeze made the Fostex sound bright. The LCD 3 sounds notably warmer, more meaty or thick. I preferred Yello (The Race y Electrified II) with the LCD3 but I preferred Brother Dege (Too Old To Die Young) with the TH909; I liked to listen to Amon Amarth (Balls To The Wall y War Machine) with the LCD3 better, but I preferred the TH909 when listening to Therion (Rise of Sodom and Gomorrah).