Well yes, here you have hit upon a variable in our preferances which definatly plays a major role in our at ear experience, where the amp matched also plays a definitive role IME...
Keep in mind, I was using the 990s as my ROCK genere application phone. Insert here also these recordings of mine, although lossless format, may have also contribuited to my experience.
Although, in direct comparison with my other phones (everything downstream remaining the same) the 990s presented as you describe in your review. Also, this finding was consistant with each of my amps, even when I preamped with my Foreplay, with what I consider to be my most rolled off HF tubes. Again, at the volume I prefer Rock to be presented, LOUDLY, but what I consider safe levels !
And perhaps this is why I identified with your assesment in your review with regards to the 990s, and others have a much different experience. Also, I will add at these levels perhaps (although I do not necessarily believe so in my case) amps may be overdriven adding to the problematic presentation of the 990s. we describe, maybe (?)...
At any rate, I stand by your conclusions, verified by my experience, relative to my other gear used, caveat being my "preferances".
Very useful discussion in which I am sure, we will be next pulling out SPL meters and such.
However, mine are real world applications for the fun phone in the Beyer lineup, I wanted to use as my Rock phone in my systems with my music... Totally superceded in every way but comfort perhaps,and even at low listening levels by my RS-2s . Which are not at all uncomfortable for myself, although do cost a considerable amount more of course.
Again, YMMV due to nearly infinite variables... However, maybe not!!!