what does "too dark" mean

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.