is that in isolation, or in mix context?
a lot of unpleasant high frequency content, that even tube amps generate, is masked by cymbals, but carry important transients that make the up most of the note attacks.
In every context.
I don't agree that there is any unpleasant, high frequency content that contains important transients. I can't think of any playing situation for which I want to retain unpleasant, high frequency content that needs to be masked by anything. To each his own I suppose. Thinking about it, maybe some metal or other specialized sounds might need that, but that's not what I play.
I have designed and built many tube amps over the decades. I also own(ed) many tube amps. Never dialed up a tone that needed unpleasant, high frequency content to get a good a sound. Sure, some amps are capable of putting that out. Many Marshall's in particular went the way of horrible, bright tones. As did many other amps. Those are the amps to stay away from or modify IMO. Also, I have never agreed that I should use additive EQ (i.e. add treble) to a guitar sound to sit in a mix properly. If it isn't sitting in the mix it is usually because of frequency congestion for which subtractive EQ (of one or several instruments) will solve the problem. All of my tube amp experiences are with guitar cabs with limited frequency response. Connect a full range speaker and you have the opportunity for even more high frequency trash.
My approach with the Kemper is to use a PA, full range type of speaker as a monitor. The Kemper is ulitmately connect to the FOH PA anyway. This gives me the possibility for a matched tone between the two. This leads to the problem of getting rid of unpleasant, high frequencies. Pure cab, hi cut, presence cut, treble cut all are necessary for me to get the sound right.
Sorry, this got way off topic from the OP.