No problem! They're more reserved in the upper frequencies than the 6xx. They lean neutral/dark rather than bright. I use SoundSource on my Mac for EQ, and I've never found myself bumping the treble down with them. Depending on the song, I've boosted the treble at 16k when I want that extra bit of sparkle. But honestly I've been listening to them un-EQ'd most of the time. And although they lean dark sometimes, I find that the imaging in the upper frequencies is still stellar. Like the hi hats in "Only" by Nine Inch Nails. Dude. Perfect. They're clippy and ticky and metallic in the best way. On my Argons, those hi hats sound like they'd been recorded in a can.
They're definitely not fatiguing or overly bright. Like I said, I've been listening to these for 3-hour+ sessions and I'm not having any fatigue at all. They're not like the Sundara where you've got a solid hour of fun before the fatigue sets in. These are legit marathon headphones.
Add on: I like them through my DropxTHX 789 more than the DarkVoice336 SE. I'm running them with the Apos Flow balanced cable from the THX 789, and they're super clarified. The DarkVoice adds some warmth, but I don't think the Caspians need more warmth.