RE: Brightness
1. Fit
Positioning and ear-shape have a lot to do with the sound. If I sort of play with their positioning, I can make them sound a tad bright. I've even made my old LCD-2 (generally considered a dark headphone) sound bright like that, by angling the cup in a certain way. The TH900's default fit on my head is fine, thankfully, but on certain skulls I can see them sounding a tad bright. I don't have any worked-out science behind this though, so consider it anectodal. As far as I can recall, the only overly bright period I experienced with the TH900 was during the first 20 hours of burn-in.
2. Digititus
Mainly source related. Sometimes it's just the nature of the record, sometimes it's the nature of the DAC. Most of the time it's a combination of both. Some people are able to counter that with a thick sounding tube-amp, but if your amplification is very linear and unabashedly solid-state in character, then it's the DAC's output stage you'll need to work on. Personally, I find my ears rather averse to the coin-in-mouth signature of Sabre DACs, whether cheaper USB-powered ones or even high-end units like the Resonassense Invicta. I'm sure there are exceptions to the Sabre implementation, but most I've encountered that don't feature this digititus tend to be very expensive (beyond $6K) units. For that reason alone, I tend to stick to wetter-sounding Wolfson or AKM based DACs, though I suppose there are exceptions to those implementations as well.
3. Origins
Nothing to do with biology or ethnicity. I'm talking about the sound your brain is accustomed to. When I still had the LCD-2, I found that switching between them and the TH900 made the latter sound a tad bright, but not overly so. Took a while for the brain to lock onto the signature, but after that it was good. If you're the type that's been using dark DJ cans his whole life (AiAiAi-TMA, ATH-Pro700, Pioneer HDJ, etc) then even the thickest, darkest "audiophile" grade headphones are going to end up sounding bright in comparison. Heck, I once heard the ATH-Pro700 mkII and even the voice of my own conscience sounded dark after that.