The funny thing is that it's now reported that all their amps (HA-160, HA-160D and HA-160DS) outputs the same 250 mW into THATSABIGMYSTERY ohms, not just the newer HD-160DS. So it's been changed for some reason. That doesn't seem serious to me. And that seems even less sensible from a marketing point of view to downgrade the rated power.
Anyway, I suppose in theory 250 mW should be enough for most high-impedance dynamic headphones below 600 ohms. Not so sure about the K1000 (still, in theory).
i also read rather good reports about pairing it with the LCD-2 or Sennheisers HD6XX, but not so with the HD800 and more demanding orthos. Could it be possible that it's now underrated for lower impedance headphones ?