I've bought the 1990 about 2 months ago and I tested it on all my DAC's and players. I didn't found the highs to be disturbing on them, except one case - Burson Conductor Air, and only with some audio tracks. It uses V5i - the sound has a relative V shape form. V6 Vivids sounds the same ( just a lot better ). So I believe this could be a reason.
On other devices, like FiiO X5II, FiiO M3, Topping D10 ( on the line output !! ), Sabaj Da2 or Burson Play ( with V6 Classic in I/V and Vivid in buffer and gain ), even on my phone, I had no trouble with the highs with same tracks. I didn't tested them with V6 Vivid in I/V in the Play but I suspect I might have the issue due to V shape sound.
However I found a trick on the internet - to fold in 2 some napkin or toilet paper and put in front of the driver. Seems to have some results. You may try it and see what happens.
Also if you have a parametric equalizer you can try to put a 6-10dB attenuation at 8.3kHz.