Please excuse any dramatic prose below, I'd just seen the Hobbit ... where darkness will soon prevail ...
"Dark" sounding hi-end equipment certainly does exist to my ears. Certainly some gear seems rolled-off (ie: early Rowland) and therefore is deemed "dark"; however at this level (top grade equipment) it's rarely a simply frequency response issue.
No, to my ears it's very much a matter of contrast, a kind of ratio, in the same vain as video contrast. Some gear tends to sound darker within the spaces behind the notes, like a pervasive black background shadow, such as Krell's early class A based amps or my Classe DR3s, which are considered dark sounding amps by many, including myself. In all honesty, I find most good Class A driven amps sway towards the dark side.
Over the last decade, to my ears at least, as more & more equipment has moved towards IC/computer technology, the result has been an overall brighter and certainly more analytical presentation. I understand that's a vast generalization, and it is, but to my ears, older discrete based equipment is consistently darker hued in comparison to current gear.
The thing is, many people will consider that overall detail always suffers at the hands of darkness, but although that maybe true in many cases, it's certainly not the rule. I've heard power-line conditioning darken a system "black" while still enlightening detail retrieval. On the other hand, the Technics 1200, despite what the many fan-boys creed, will generally always provide a consistently dark and narrow tonal picture - dark; but void of inner detail.
Amps are one thing, pre-amps and source components may also fit within a contrasting scale. Lyra for example, is rarely considered dark, but the latest Ortofons might be, especially in comparison.
Certain colorations have long been considered a "negative" thing within most audiophile circles, but to me, that's simply being far too dismissive of the available hues good equipment may provide. Dark & bright are simple words that many people simply take for granted ... a dark mention will often be perceived as "too dark", same as bright, ... but do these terms truly provide you with a picture of the components true musical delivery?
I don't think so ...
tb1