FINAL VERDICT
Comparing a pair of new monitors when you've been using the same ones for years isn't easy. I've been using Yamaha HS's for at least 10 years now and was used to their sound.
Why did I want to change?
I felt the guitar tone was always a little too unbalanced in the high frequencies, definitely more than I hear with the real amps I have, so somehow unnatural.
Why the Adam?
Based on YouTube comparisons, I felt that the Adam's sounded less shrill than the Yamaha, so it was worth a try (read my first post in this thread)
What I noticed
The Yamahas are definitely more unbalanced in the mid-high and low frequencies. If you're used to this sound everything else will sound less detailed, less precise, but it's an overdrive on these frequencies, not something positive. With prolonged listening the Yamaha's strain my hearing.
The Adams are definitely more balanced in comparison. High and mid-high tones are more pleasant and don't tire the ears, plus the midrange is more present, which is almost lost on the Yamahas.
Presets and amp models
I confirm my initial thought, I know room and acoustic treatment are part of the equation, but in a well treated room like mine, all things being equal, the difference in EQ of the two speakers is clear, so a preset or model created on the Adam will tend to be too bright on the Yamaha and one created on the Yamaha will tend to be too dark on the Adam. I'll add a wish.
Side thoughts
I sold the Kemper and switched to Fractal because I felt that many of the profiles sounded too bright, different from the actual amps. I'm afraid the reason was really because of the speakers, but well...it was the way to approach the Axe-Fx and I'm totally happy now.
Final decision
I'll keep the Adam's. I prefer the sound of these monitors and they tire me out less. I find it closer to what I hear with headphones and the end result I would expect from a modeler, closer to real amp tone.
Not that Yamaha's are bad, thousands of people choose them (and they are not stupid, I'm not smarter), just look online, on social networks, they are everywhere in the photos of home studios, they are a great product for the price, but they are addictive to a type of listening that is not perfectly accurate.