I have a hard time believing this. When I plug my Hifiman HE6SE V2 into my Darkvoice 336SE, or my Little Dot MKIII, lowest bass frequencies disappear and upper bass frequencies are barely audible. This frequency response makes music awful sounding is unlistenable to me. However when I plug the same Hifiman HE6SE V2 into a low impedance friendly headphone amplifier such as my Schiit Jotunheim 2, the lowest bass exists again and the upper bass returns.
Tube amps don't really count