It's a very faint effect. Sort of a veil but instead of blanket that muffles the sound think of a fine translucent metal mesh that throws a extra sheen over the music. It is not actual brightness but something else. It could be that some frequencies become louder again when they have completely died out.
Most likely cause that driver keeps resonating longer than it should with some frequencies. This sort of behaviour has been documented with other planar headphones.
Here is spectrum plot of Hifiman HE400se. It does indeed look like "some frequencies become louder again when they have completely died out".
Here is Sennheiser HD800 for comparison.
Hifimans plot looks like a mess but it should be mostly inaudible. Maybe Tres 16ohm has similar behaviour but with my pair it has crossed audibility treshold. Having a messy spectogram does not mean a headphone is bad. Most people would not say that $7000 Susvara is bad, would they? Here is Susvara's spectogram.
Spectogram plots are from:
https://diyaudioheaven.wordpress.com/
So I would not worry too much about that "hiss" with 16ohm Tres. It might be thing only with older units or just my particular headphones.
Haha! That is a bulletproof plan
They're both great. I propably had some seal issues when I measured the 16ohm. It should have more bass than graphs suggest.