So I had the chance to try out both the Fostex TH900 Mk II (stock pads and cables) vs. the Denon AH-D9200 (also stock pads and cables). I intentionally kept both at stock configurations because I figure if you're going to be spending over $1000 on headphones, they should be designed well enough without having to mod the hell out of them for a good sound. Thankfully, both sound great with their stock cables and pads. I didn't use a fancy amp/DAC to be honest (Sound BlasterX G6 since I'm more into gaming than music), but again, I found these headphones very easy to work with.
I managed to snag a new Mk II for about $1200, and I got the Denons for about $1600. The two headphones are very comparable, but here's the TLDR version: The Denon 9200's are great - but their price tag vs. the TH900 doesn't justify the purchase.
I tested both headphones for both music (mainly movie soundtracks and 70's/80's/90's/early 00's Rock music) and gaming (RPG's, FPS, RTS's, atmospheric games, etc.)
Some specifics:
Bass - The Denons have very tight and controlled bass, can really handle the lows very well, but I just found the Fostex to be deeper feeling and just more fun. You FEEL more with the Fostex vs. the Denons. Both headphones handle deep bass like champs.
Midrange - Both perform well in the midranges. I think the Denon is bit more technically precise, given that they're closer to reference headphones (as others have stated) than the TH900, but I didn't notice a massive difference (again - it doesn't justify paying $400 more). Neither headphones' bass frequencies bled into the midrange.
Treble - I will admit, without proper EQ, the TH900's treble is exhausting. The Denon's are better out of the box when it comes to those piercing highs. However, with the right EQ, I was able to control the massive V shape of the Fostex's and bring them more in line so that the highs don't assault the ears. For gaming, the V shape is better - for music, not as much depending on what kind of musical experience you're looking for.
Surround sound - The TH900 sounds more *open* and a bit more clear than the Denon's do. Perhaps this is because of the semi-closed design, but the sound just sounded "bigger" with the Fostex's than the Denon's. In FPS games like Call of Duty, I found that the Fostex was more accurate in positioning than the Denon's. They seem to be better designed for surround sound than the 9200's. I was able to tell the location of people above me or behind me more often with the TH900 than with the Denon. I tested both pairs of headphones' surround sound capability using Dolby Atmos for Headphones, and DTS Headphone X. In a game like Jedi: Fallen Order, the clarity of the TH900 brought out the ambience slightly better than the Denon 9200. I felt slightly more immersed with the TH900 than the 9200.
Portable devices - Both are very easy to drive. I was able to play each of them on a regular Sony mp3 player with no issues. No amp needed. For the Fostex, however, I needed an 1/8" adapter to plug it in, while the Denon conveniently had a dual plug in at the end of the cable. You can simply remove the 1/4" plug and the 1/8" is under it. Very convenient.
Comfort and looks - The TH900 wins hands down. They're more comfortable to wear, and I guess I'm just partial to red, lol. The bamboo of the Denon was nice, but the TH900 just looks and feels better in my opinion with that red birch cherry wood. My main gripe is that the cable length of the Fostex is too damn long, while the Denon's are much more reasonable.
Overall: The Denon AH-D9200 sounds really good, and if you're looking for a very precise and controlled sound, it may edge out a tiny bit over the TH900, however, I just don't think that little bit extra precision is worth the extra $400 you would pay vs. the TH900 Mk II's, especially because you can rein in the TH900's treble with the proper EQ. That's just my opinion, and I know these things are really subjective, but I hope this helps. I wanted to give back after asking so many questions on this board about all the different headphones out there
Thanks everyone for all the help and advice!