@potterma: plenty of headphone amps can drive the HE-6 to very loud levels. The Hifiman EF5 can do it, fer gosh sakes! I'm quoting your phrase "what it takes"-- and saying that "what it takes" must be more involved than power levels.
You might have noticed a few pages back someone said that the Zana Deux can't do justice to the HE-6. The Zana Deux has probably several watts of power. Remember that most headphones only need milliwatts at normal listening levels, and my estimate is that the HE- 6 is only about 10 dB less efficient than the HE-500. A watt should make it very loud, and the EF5 demonstration proves that.