Due to the noise floor, the DAC-100 does not mesh well with low impedance high sensitivity headphones. Any other combination of impedance/sensitivity is fine though. Due to the current limit of the DAC-100 though, low impedance headphones will tend to clip the amp at high volumes.
I have an HE-6 and it sounds good with the DAC-100. It doesn't have a ton of headroom, but it's enough to hit 100dB. A while ago I had compared against a Lyr and an LCD2. I preferred the LCD2 with the Lyr, but sonically preferred the HE-6 with DAC-100 more, even though the Lyr had significantly more power and headroom. I would feed the DAC-100 into a speaker amp to drive the HE-6, or in my case into the HAP-100 preamp then into a speaker amp.
Planar magnetics in general don't care too much about output impedance interaction since their own impedance curves are practically flat across the audible range. There may be slight differences in electrical damping, but most people will never be able to tell the difference unless you make a habit of listening to harsh test tones. The Fostex T50rp sound just dandy from the DAC-100.
Thanks Armaegis for the information. :)
I've thought about the LCD-3 and HE-6. I'm probably going to get an electrostatic amp for STAX headphones as my next major purchase, but that might change. I could use the DAC-100 as my DAC to feed into the electrostatic amp. Unfortunately, electrostatic amps seem to be rather expensive, the cheapest one I've seen recommended is from STAX and still a few thousand dollars, but, such is life. The DAC-100 is still an excellent pairing with the HD 800, which I don't plan on selling anytime soon.