The original comment was in the context of small amps like the Apogee Duet or IFI iCan. I can't find a power rating for the Duet but the iCan is rated at 400 mW into 32 Ohms. A 300+ Ohms load like the HD600 is doable, but will definitly drop that power output into double-digit territory. And if you talk about something like the Fiio E17 or iPod with no amp, just forget about it.
Technically higher is better, but in the end it's what we hear that matters. I was choosing between the iCan and the Asgard 2(380mW into 300ohm), so i went to listen to the A2 since a local store had them. I even brought my current gear there so I can compare the differences. All I could distinguish within that limited time and using what limited music I had was wider soundstage and slightly better bass response which was good, but I couldn't justify the price tag for them. In the end i got the iCan because it's almost comparable to the A2, has attractive functions, and I had an opportunity to get them at 2/3 of the retail price brand new, though i got the older version of it.
Edited by Ophiel - 8/8/13 at 8:53pm