I see a lot of variance on people’s amp recommendations for products. It seems to me like a clean 1W into 32Ω is enough to drive the majority of headphones to 120db, which is extraordinarily loud to the point of being unlistenable. Tons of middle-tier desktop amps can handle going over 5W per channel at the same impedance and some even go over 10W. Are these wattages actually useful for anything? What wattage would set you up for any headphone, regardless of budget?
Doubling voltage increases loudness by 6dB. Doubling of power (watts) increases it by 3dB, but 2x voltage = 4x power.
HE6 is 89.7 dB/V SPL so 4V would be 101.7dB.
It’s not just about voltage, though, it’s also about current/power. Voltage is only really relevant for high impedance headphones. Low impedance, most amps will run out of power before they run out of voltage.
4V @16Ω is 1W. I have several little dongles here that have no problem doing 4V @300Ω into a HD600 or HD800S (53mW) but they can’t do 1W.
Most headphones, you don’t need 1W though. HE6 is a bit of an exception.