@verdantaudio I know that you have done extensive testing with DACs and I have read almost everything you wrote here about it. It was very informative. I believe what you are hearing today with the GAN1.
I have done "amp rolling" with my RAAL SR1a earphones over the last 2 years. I have a very high opinion of them, and I wanted to find the best gear for them. I tried 10 2-channel amps with them, $2K - $11K range.
One thing that was a factor in my evaluations was my physical reaction via ear fatigue. I get fatigue easily with bad sound and the SR1a can bring that on with a bad pairing. The only amps that did not cause any fatigue were the KRELL 175XD, CODA #8, and Parasound A21+, I attributed that to the slight warmth of the amp, preamp, and DAC.
The GAN1 trumped those 3 amps with the SR1a, it sounded the best. Thus, I do not think of it as bright. My ears would have been screaming, especially since I was testing with hard rock/heavy metal to purposefully try and cause ear pain.