Thank you for your detailed reply. I have a HE-560 and I believe it needs more power than the Atom can provide.
I hope
@tomchr can release more information about HPA-1.
Just used few minutes ago a 1KHz sinewave on my HE-560. I could barely listen to 1V RMS for few seconds; hope tinnitus will spare me this time.
I used the same volume knob position and played regular songs (average to high compressed songs, 2019 pop music) and found that knob level needs to be increased. With cans out of my head I played the same 1KHz sinewave and I measured 2V RMS with my scope. When switching to highly dynamic Chesky music I needed to increase volume even further, somewhere where 1KHz sinewaves will get to about 3V RMS.
My personal conclusion would be that if an amplifier was designed to deliver the max. output power for an input of 2V RMS, then with music playing it might never get to full power, especially with dynamic music with peaks that barely get to -3dB. Probably this is why most amplifiers are delivering full power for an input of 0.7...1V RMS.
So, in HE-560's case, definitely Atom can drive HE-560, but also any headamp able to deliver 3-4V RMS on the 6.3mm jack for 1V RMS on its RCA inputs (a gain of 3...4X).