I was feeding it with a Schiit Modius and lately with a Topping D30 Pro. Both can output 2V IIRC correctly, and sound equally great to my ears for less sensitive headphone use.
Note that in the HE-6se review, it rated the headphone output at 94 dbSPL at 1.37V, which is at ~30 mW. The max output voltage of the amp is ~2V + 6db == ~8V, so for the sake of argument call it ~100db @ ~120mW max. I'm a quiet listener so I never even get close to 100db on peaks, even playing uncomfortably loud for me. The other listeners thought it was fine, but I thought I'd do the computation to make sure you went in with eyes open.
You can see why people want to use speaker amps for this headphone though if they want to rock out at super volume.
As a more specific contrast, the A30 Pro has 14db gain max, plus 6db if you use XLR inputs with a typical DAC which outputs 2x the voltage/6db gain into XLR, so ~2V + 20db total, i.e. ~114db @ ~2.5W max for your peaks, so you could play an
average of 94 db or so and have full headroom (which is supposed to be ~20db), which is crazy crazy loud.
On the quieter side of things, for the Shure SE-846 IEMs I use, to keep the volume reasonable, I have to turn down the analog volume knob on the A50s close enough to the bottom to start hitting the uneven balance part of the pot
and turn down the digital volume control on the D30 Pro. I try to avoid losing digital accuracy, but I got the D30 Pro for the amazing range it has and it certainly sounds great subjectively.