what does "too dark" mean

>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.

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.