I can't go into a full explanation. The Valhalla is not bright or sparkly (unless that's on the recording) and it does not act as a tone control. Also, particularly with minor mods and good cables, the HD650 roll off is less than reputed. But what it does in the treble on my HD650s was distinctly more accurate and pleasureable than on the Fireflies and even the Lyr (cables might have made a difference, but all were good aftermarket ones.) Power specs are of limited use, so long as they are not drastically short of phone recommendations. And I didn't say anything about "limits" the naturalness. But I did compare the triode and single-ended aspects of sound to power amps with those features: triodes are smooth and uncanny in their realism with realistic transients but some is illusion due to a bit higher harmonic distortion perhaps. Single ended similarly increases the even order harmonics which are shown in studies to increase perceived realism. On some music passages, this can reduce the vividness compared to, say, the literal but sometimes exaggerated sharpness of solid state, for example. I'm sure others can put these better into words or context than my tired brain at the moment.
