I've been spending a lot of time listening to my music gear (especially the Darkstar and Decware CSP-2/Taboo amps) lately, so I've been kind of away from the forums. Now, here's the deal: The Fostex th900 sounds kind of odd - and with some background hiss - out of the Darkstar. Definitely not a match made in heaven. The music feels kind of disjointed and claustrophobic...It feels like a gecko (the th-900) trying to wrestle with Godzilla. This is one in which too much raw power leads to nowhere. Conversely, as I've remarked before, the Hifiman EF-6 amp - a rather powerful amp as well, but not as powerful as the Darkstar - with the th-900s is (to my ears) a match made in heaven, although I have it connected to my Headroom Ultra (non-balanced) headphone amp, which is serving as its pre-amp, in order to have better control of the volume since the volume knob of the EF-6 is a monstrous stepped-attenuator type that offers very little control over the volume (note: by the way, the th-900 driven by the Headroom amp sounds amazing as well). It must be noted that there is still a very slight residual hiss with the EF-6 (not surprising given the high sensitivity of the Fostex headphones), but once the music starts playing...behold, the promised land.
Thanks for the extremely detailed impressions about the TH900 pairing. I also was interested in the HE-6 pairing with the Darkstar since I have the HE-6s too and have been contemplating between a power amp to drive them directly or a dedicated headphone amp. It seems to be worth posting in the Amps for HE-6 thread as well (if you already haven't done so yet).
The EF-6 would make a decent amp for both the TH900 and the HE-6s, in my case, so that I can reduce the number of amps needed to drive both down to ONE! Will hope to find someone with an EF-6 I can get a chance to listen to locally.
Many folks have also commented on the TH900 + DNA Stratus pairing, but not sure about HE-6 with Stratus since they need MOAR POWER!