I'm personally sensitive to around the 8k area, perhaps somewhere north of 10k as well.
Long story short - I can listen to the Roxies at high volume for like 8 hour stretches. I never feel fatigue. It doesn't matter what I'm listening to. It's weird. The only thing I can think of that had this quality for me probably were a set of ATH-L3000s I used on a daily basis for a year and a half.
Me too. My ears are really treble sensitive too. I don't mind treble as long it's distributed evenly long the spectrum. I only find it hard to bear when there is huge peaks here and there which is what normally takes aways the smoothness IMO.
Sounds like your source pairs with roxanne really well (or your bass gain is higher than mine). :) I haven't listen to any warm source/amps for a long time maybe that's one of the reason I found roxanne can lose its "emotions" after long periods of listening. And also 95% of the songs I play contained vocals (well, badly, accurately, falsely mixed), which is probably why I have really high standard for vocal performances.
Strangely, I really enjoy piano performance on roxanne. I think the tuning is very suitable for it.