what does "too dark" mean

when you describe a preamp as being too dark what does this refer to?
I would recommend J. Gordon Holt's Sounds Like audio vocabulary compendium.

Dark: "A warm, mellow, excessively rich quality in reproduced sound. The audible effect of a frequency response which is clockwise-tilted across the entire range, so that output diminishes with increasing frequency."

If you are interested in learning more, I have several links in my website's audio section:
Val e-diction
I take it to mean that it is thicker in the bass with a kind of closed in feeling. Vocals and soundstage presentation are probably more recessed. Kinda like a brooding emotional feeling. Probably will not hear "air and space around instruments" as much, and decay and detail may be limited. Just my take on it, but everyone probably defines it differently.

The opposite of "too bright."
It probably means that the person speaking likes his sound on the warm side of neutral or, that he likes the top end rolled off, but that what he is hearing is too warm or too rolled off FOR HIM. Remember dark to some means warm and to others it means rolled off. You won't find it in Webster.
I like the above recommendations and reponses.
To keep it simple however, "too dark" means "DOESN'T SOUND GOOD!"...just like "too burnt", "too sick", or "too ugly" have negative connotations attached.
I would call its opposite "too light" or "too white." Not "too bright" as tbg says (in my book, too bright's opposite is too soft or too dull). Very subjective, and JGH's glossary is useless IMO. I think you can have a dark sound without being rolled in the highs as he suggests.
Potentially deep and rich sounding, which are usually considered positive attributes.

Dark sounding can usually be interpreted where the lower regions of the frequency spectrum are reproduced adequately, but the higher frequencies are not produced with the same weighting. Thereby reproducing what appears to be an unbalanced presentation.

Or more commonly as Drubin put it, rolled off highs.

"too" anything is no good. That being said, this audiophool can handle "too dark" much better than "too bright."
Warrenh, a true pearl of wisdom! Nothing irritates me more than the shrill shrewish screeching of a "too bright" system. Well, maybe I'm overstating, since most Agoners have systems which wouldn't send me running out of the room, even if leaning toward the bright end. I, like Warrenh, would be more at ease with a top end that is relaxed vs. one that is aggressive. But, as Paul Simon said,

"One man's ceiling
Is another man's floor."
Clearly this difference in opinion demonstrates the unfortunate truth that these terms are meaningless because they convey no information. The opposite of too bright is too dull, damped, excessive treble or no liveliness. Dark has always meant something else to me its meaning is almost impossible to properly define its like trying to tell someone what an apple tastes like.
I'll try its as if the timbre and texture of the sound is off in a deficient way so that the sound is closed in and collapsed (note not compressed) on itself. I agree that there is an element of boxiness.
Think of a portrait photo~on a dark portrait,details you know exsist,fine lines,uneven textures,lashes/brows blended would only just be visible in the most illuminated areas(foward points)outlines/edges less defined,full shape obscurred,background fading to inky black.Seems dark would lack stark detail,less 3 dimensional appearance,yet depth could remain in a raised relief,like a dessert sand dunes after sunset.Mind that this is just a guess based on what "dark" means to me concerning an image/picture.
A preamp that is to dark has a black faceplate rather than silver or white.
I agree with Red2 and Mechans. They hit it on the head. Closed-in, overdamped, air truncated, ringing or vibration damped down. Not my kind of sound...sort of dead....
>Or more commonly as Drubin put it, rolled off highs.

Actually, I argued against that as a hard and fast rule. I think you can have extended highs and still have a dark system. My personal vocabulary, YMMV.