At this point, I only had about 30 minutes or so to test the Amiron that my friend owns. This was connected to his equipment as well. He has the Xonar STX for his DAC, and is using a Schiit Asgard 2 for his headphone amp. In my brief time in testing though, I can say this headphone sounds far more refined and articulate than the DT880 Premium 250 Ohm that I used to own, but sold. It is also a big step up from my K712 Pro that I currently own now. I will be buying my Amiron Home in a few days, and I will then have to wait a few days for it to arrive in the mail. I will give my impressions though compared to my K712 Pro, once it arrives. Quite frankly, I don't see how anyone could have any issue with the treble on the Amiron. In my brief testing, it was very clean and detailed, but did not have any element of harshness or sibilance to it at all. This headphone is quite an improvement in every way from the DT880 and T90. I saw the review that Metal571 did on the Amiron, and he claims that it has terrible brightness issues with the treble. IMO, there is just no way. I am 100 percent convinced that he reviewed a defective pair. This is a very smooth sounding headphone in the upper frequencies. I think you should buy a pair, just like I am doing.
I bet you will be impressed.
Edit: I think you may have been confused about my DAC. My DAC is the Creative ZXR, which is a PC sound card. It has the Burr-Brown PCM1974 DAC for the front channels, which is a 127 SNR part. It has the Burr-Brown PCM1978 DAC for the other channels. This is a 124 SNR part. I connect my M-Stage HPA-2 headphone amp to the front RCA line outs on the sound card.