All this talk about HE-6 power requirements gave me an idea for a test that would show how much power is used in my system for a particularly demanding recording. Using the recording listed below, at 2 minutes 25 seconds into the song, I snapped this meter image from my Yamaha MX-1000 that shows a momentary peak reading of 24.2w.
At the same time I noted a peak reading of approximately 100 dB using on my analog RadioShack Sound Level Meter. Here's a pic of the test setup.
As a point of reference, without changing the volume level, I played a 1 KHz / one third octive warble tone at -20dB from a Stereophile Test CD1, and recorded a level of 94 dB, so the peak when playing the song was 6 dB higher than this reference tone.
Even at these levels the sound was clean and effortless. I'm sure the HE-6 would play even louder, but I wouldn't even attempt to try anything louder for fear of damaging my hearing. I rarely listen to music at these levels, but will occasionally, if only for brief periods. Note, my average listening level is about 75-80 dB.
CD: Henrik Freischlader Band Live (Disc Two) Track 3 - Drum Solo by Dirk Sengotta
Keep in mind that your power meter display is calibrated for 8 ohm load and not 50 or 60 ohms of your HE-6. Your real power into the cans is more like 3 watts.