I mean some transducers are just more efficient than others. That's usually something listed on the specs of the headphone. Innerfidelity used to have a whole bunch of headphone measurements and one of the things they did was measure the output at both 1 mW and 1V. But that site seems to have gotten absorbed into stereophile and I can't find those measurements any more.

This site now has a bunch of them:

https://diyaudioheaven.wordpress.com/headphones/measurements/
For instance you can see a really hard to drive planar (74 dB/mW 86 dB/V)

https://diyaudioheaven.wordpress.com/headphones/measurements/hifiman/susvara/
And a really easy to drive planar (103 dB/mW 120 dB/V)

https://diyaudioheaven.wordpress.com/headphones/measurements/audeze/lcd-x-fazor/
You can use those numbers and then add 3 dB for each doubling of power from that 1 mW reference. So to get the Susvara to the same output as the LCDX, you have to get 29 more dB. If we round that up to 30, you've got to double the power 10 times. So you need ~1 watt at the impedance of the susvara, which is 60 Ohm. The Heresy specs at 1.6 W at 50 Ohm, so you're probably there, to get 103dB output. Considering they spec 2.4 W into 32 Ohm, and Amir's measurements bear out that you can get that before clipping, the 1.6 into 50 Ohm is a good bet.

The Atom specs 1 W into 32 Ohm, so you won't be able to get as loud into the Susvara before clipping.

All this said, if you're buying something as crazy as the Susvara, you can probably swing a bigger amp than these $99 guys.

If you're running the HE560, or LCD2, or LCDX, either of these amps will melt your ears.