Well, I have only had the LCD-2 for a few months, but still think it's dark sounding. I might be selling it before selling the HE-6 ...
I agree on your synergy point - whatever that synergy thing may be - is something that could be an important factor to why different amps make a headphone sound different. Output impedance is of course an important issue in some cases, but not regarding the HE-6, since it's impedance curve is more or less ruler flat (if I understand this correctly).
It could be interesting to know what that "synergy" is all about concerning the amping of HE-6 




















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