But surely using a different dac/amp combo, couldn't possibly produce more than subtle differences in the harshness of the treble, with the headphones themselves have a slight treble spike?
You'll be surprised what changing the type of DAC can do to sound, implementation of DACs matters a lot, but I've usually been able to tell if a DAC is a ESS Sabre or a Burr-brown, they have different sound characters, ESS Sabres seem to be more detail and soundstage/imaging oriented and burr-browns tend to be more dynamic/robust with smoother highs(on the better ones, the crappier burr-browns are often quite sharp sounding up top). You would think it wouldn't, but the change can be the difference of tolerable to not tolerable. I think it's more of a treble texture thing. Honestly I usually don't like the Amirons or honestly Beyers in general out of anything but a R2R dac or a nice Burr-brown dac in terms of DACs. Other sources such as vinyl, cd players, etc. is a different story.