what does "too dark" mean

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.

Clearly.

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....