So today I recieved my pair of Fostex TH900. I put them through the DFR and a 6mm to 3.5mm adapter. It sounded like absolute garbage, and I was so disappointed. Switching back to the D5Ks, I realized that the contact was poor because of the adapter. So I called up a friend of mine and got to test it through his entry level schiit stack. Sounded much much better and I was very relieved. He even was so kind he lent me his Magni 2 which I am now running through the DFR.
Interestingly, the DFR seems a bit brighter than the modi, with less bass impact but much better soundstage and dare I say clarity. The TH900 resolves like no other headphone I have ever heard. I honestly did not expect such a big jump up in resolution coming from the D5K which sounds awesome to my ears.
Comparing the two briefly through entry level equipment, I will say that the TH900 is several levels above in pure sound quality and resolution. The bass quality of the th900 seems much better, and th900 handles different genres much better than the D5K. I would, however, not be surprised if someone prefered the D5K in some genres such as house, dubstep etc simply because the bass is more boomy - it feels more like a sub woofer in most of those tracks. Unfortunately for the D5K, that also means it muddies up some other genres. A good example for this is the band/artist Tycho. Their songs gets a bit too muddy for my liking on the D5K, while in the TH900 it seems absolutely perfect.
The TH900 to me seems like a much more audiophile friendly pair of headphone than the D5K. The D5K sounds more warm and cozy and just based on initial impressions, seems like something I can listen to for longer. I remember when i first got the D5Ks and I had issues with fatigue, which went away.
My single biggest issue with the TH900 is the listening fatigue Ive acquired today. Maybe its the natural brightness of the headphones paired with my bright and entry level equipment - or maybe its something I need to get used to, just as I did with the D5K. But right now I am having issues enjoying the music because my head hurts. I think some equipment (amp+dac) that makes the overall sound a tad darker would be perfect.