The Fostex TH900s and Denon D5000s are very similar in timbre due to similar design. They both have a V-shaped sound signature but the TH900 is closer to neutral. If you really enjoy the Denon D5000s and want to upgrade to something with higher quality sound with a flatter frequency response then the Fostex TH900 is ideal.
Comfort: Denon D5000s are more comfortable but only by a small margin. They are lighter and have less clamping force than the TH900. TH900s have slightly more room inside the cups. The D5000s and TH900s are both one of the most comfortable headphones I’ve ever worn.
Soundstage: Very similar. I'd give an edge to the Fostex TH900, soundstage seems a little wider, deeper and has better imaging. Both headphones have fantastic soundstage for closed headphones.
Treble: The Denon D5000 treble is a more pronounced and brighter. Switching back and forth between the D5000 and TH900 makes the D5000 treble sound a bit too harsh and unnatural. However, after using the Denon D5000s for longer periods I get used to the treble emphasis and don't find it so harsh. The treble quality on both the D5000 and TH900 are top notch and have fantastic extension. TH900s are a winner in the treble department due to better balance and integration into the rest of the sound spectrum.
Mids/Vocals: The Denon D5000s mids are more recessed in comparison to the TH900s. Vocals on the TH900s are much more rich, detailed and upfront than the D5000s. Vocals on the D5000s have more airiness though (perhaps due to the emphasized treble and less mid-bass). The Denon D5000s mids are very good in their own right but the TH900s are better by a large margin.
Bass: Denon D5000s have more sub-bass emphasis while the TH900s have more mid-bass emphasis. Due to using the D5000s for much longer I’m more used to the D5000s bass so the TH900s mid-bass sounded a bit slow and bloated at first but after a while it sounds more natural. Both the D5000s and TH900s have exceptional bass extension and hit the lowest of the low notes like a subwoofer. I have not heard any other headphone that matches the D5000s and TH900s bass. In terms of bass quality I’d call it even between the D5000s and TH900s.
Other notes: Instrument separation and dynamics is better on the TH900s. Both headphones are relatively easy to drive. The sound straight out of my iPhone 4 is adequate. Using my Fostex HP-A3 DAC/amp significantly improves dynamics and gets volume high enough for movies and games which are usually mixed much softer than music. The Fostex TH900 improves more with a DAC/amp than the Denon D5000. Both headphones are forgiving to low quality music and sources.
- Comfort: D5000 > TH900
- Soundstage: TH900 > D5000
- Treble quantity: D5000 > TH900
- Treble quality: TH900 > D5000
- Mids quantity: TH900 > D5000
- Mids quality: TH900 > D5000
- Mid-bass quantity: TH900 > D5000
- Sub-bass quantity: D5000 > TH900
- Bass quality: TH900 = D5000