All orthos in general have quite smooth treble. In particular HP1 has treble so smooth that lower & higher treble all appear rather same. If visualised, similar to a ray of light.
HP50A has smooth upper treble but comparatively rough 7-9khz.
YHE50A is really good with treble. Very very clean , smooth, little too peaky >10khz but great sparkle.
Why isn't this little yamaha popular here. I think it offers one of the blackest backgrounds.
woha, my HP-50a (stereo) are anything but rough at 7-9, mine roll of exactly there so violins don't tend to shine quite as much as I like.
But, once you remove the paper-disc in front of the driver they tend to get aggressive, so I let my discs in place.