Not to add to the noise, but above comment can be misinterpreted and that may not have been the intention.
O2 or any amp for that matter need not deliver clean 1 watt power at whatever resistance they may be asked to. They need to be clean at their stated specs. Before the orthos became mainstream 1W may have been considered more than plenty. Orthos do not yet rule the planet :)
Agreed. 1W @ 16-32ohm is already way too much for most of my low impedance headphones. It's these pesky Orthos keeping me from just using one amp for all.