I think what is needed is "current capability", i.e. how easily can the amp dump current so it can keep up with the transient (after all, in a dynamic or ortho it is current that makes the sound). Has to do with the output impedance of amp, and, usually, the bigger the amp the lower the output impedance, and since the amp's usually don't show output impedance spec on the decal, we use the amp's wattage to "gusstimate" the output impedance.
Here is my guesstimate: per my experience, I want the amp to be able to safely dump at least 20x the I-max to a safe load ( I-max is the current used by the phone at a loud max volume), so I can make sure the amp is not stressed at peak transients. In the HE6's case, I will use 1W as the desired max output, the I-max is about 1/7 amp. Times 20 it will be about 3 amp. So I want the amp to be able to safely dump 3 amp all day long. To use a amp that is rated to drive 4R, I will need 35W per channel. not that the phone will use the 3 amp current, but the amp must be able to do it if needed to.... Just a ball-park guess.
It's the same thing... Divide power (watts) by resistance (ohms), take the square root of the result, and you get the current (amperes). Providing current and power go hand-in-hand. If an amplifier can't drive the amperes, it won't drive the watts either and vice versa.
I see only 4 potential explanations:
- The HE-6 sensitivity isn't really 83.5dB at 50Ohm and 1mW.
- The HE-6 impedance isn't flat, and it goes under 50Ohm.
- Amps with a rating of 1W that we tried don't actually perform well at power levels anywhere near that.
- I'm missing some chunk of physics here.