Apologies if this question has been askedad nauseam,this is my first post in the Sound Science section so please be gentle with me!

I've been into headphones for a few years now but am still somewhat mystified by how much power / headroom is required to obtain optimum performance.

With loudspeakers, things are somewhat easier. You look at their impedance (which is typically between 4 and 8 ohms nominal) sensitivity (Xdb for 1w at 1m), consider the manufacturers recommended power range (i.e. use with amplifiers between Y and Z watts per channel RMS), and depending on how loud you listen at and how much headroom you want above that, you go shopping for an amp that at least meets or preferably exceeds those minimum specs. (OK I'm oversimplifying things but you get the idea).

With headphones, things seem a bit more ambiguous. With such a wide range of nominal impedances (from below 16 ohms to above 600 ohms), sensitivity sometimes expressed in Watts and other times expressed in Volts, the fact that some headphone amp manufacturers provide maximum wattage specs whilst others provide RMS wattage specs into a small sample of loads, and some expressing it in terms of max or RMS Volts instead of mW. I find it very confusing to figure out just how much power and headroom a particular headphone requires to perform optimally with full dynamic range, and exactly how much power and headroom a particular headphone amp can provide into a specific load.

To give some examples, let's compare two Class A designs, the Schiit Asgard and the Arcam rHead:

Asgard 2:

Maximum Power, 32 ohms: 1.0W RMS per channel

Maximum Power, 50 ohms: 1.0W RMS per channel

Maximum Power, 300 ohms: 380mW RMS per channel

Maximum Power, 600 ohms: 190mW RMS per channel

rHead:

Maximum power (both channels driven)

2.00W, 16Ω load (5.7V RMS)

1.10W, 32Ω load (6.0V RMS)

0.13W, 300Ω load (6.5V RMS)

As the rHead quotes an RMS voltage figure, I am assuming the Watts figure is also RMS, or might it be peak value? The rHead doesn't quote a figure for 600 ohms, but since the trend seems to be half the power for double the impedance, it will probably be around 65mW. So into a 32 ohm load the Asgard and rHead have near identical power outputs, but into 600 ohms the Asgard is 3x more powerful than the rHead.

Let's take another example, the Schiit Vali 2 and the Audio-Technica AT-HA5050H. Not exactly an apples to apples comparison because one costs <$149 and the other costs >$5k, but it's the power specs I'm most interested in:

Vali 2:

Maximum Power, 16 ohms: 1300mW RMS per channel

Maximum Power, 32 ohms: 1000mW RMS per channel

Maximum Power, 50 ohms: 650mW RMS per channel

Maximum Power, 300 ohms: 270mW RMS per channel

Maximum Power, 600 ohms: 140mW RMS per channel

AT-H5050H:

Rated Output (20 Hz – 20 kHz):

125 mW + 125 mW (16 ohms load)

62 mW + 62 mW (32 ohms load)

31 mW + 31 mW (64 ohms load)

3.3 mW + 3.3 mW (600 ohms load)

So into a 600 ohm load, the Vali 2 provides more than 40x the power of the AT-H5050H. If I compare the AT-H5050H to the Valhalla 2 or Mjolnir 2, the differences gets even bigger, as these two Schiit amps put out 125x more power into 600 ohms than the AT-H5050H.

So, if we have 3.3mW RMS provided by one headphone amp and 450mW RMS provided by another headphone amp, just how much power and headroom is really required to drive a typical 300 to 600 ohm dynamic headphone cleanly with no distortion? This is something I'd really like to understand. (I'm referring to headphones such as the 300 ohm Sennheiser HD600/HD650, 300 ohm Sennheiser HD800/HD800S, 420 ohm Audio-Technica AT-ADX5000, and 600 ohm Beyerdynamic DT880).

Many thanks.