Burn in question and evaluation before burn in


We all experienced sound transformation before and after a new equipment or cable is burned in, however, I am wondering if there is a general rule as to which direction any burn in would be heading? Specifically, I am interested to know would sound generally go smoother/darker or brighter/more transparent after burn in? I am thinking if there is such a rule, it would be valuable to know for evaluating products.
wenrhuang
I think we are in full agreement about capacitors - see what Douglas Self has to say;

....well-known capacitor shortcomings such as dielectric absorption and series resistance, and perhaps the vulnerability of the dielectric film in electrolytics to reverse-biasing. No-one has yet shown how these imperfections could cause capacitor audibility in properly designed equipment.

The last sentence is a key one - "in properly designed equipment".

I think we also agree that it is not safe to assume that everything out there is designed properly and burn-in is necessary for many designs.
Cyclonicman,

My preamp was freshly installed with new upgraded board, it is supposedly better than the old version, but no, I have no idea how it will eventually sound.
Demo gear fully broken in and then decide if you like it. If you like it buy it...maybe even try it at home:)
what prompted me to ask the question was that I was frustrated that my new solid state preamp sounded good at first install, but became too bright, and strangely, the tight bass was gone too after a week.

Something is not right, IMHO. Check your interconnects and speaker wires. Any chance you got phase reversed - or did you flip the balanced/unbalanced switch or change the input volume offset by accident? Remote controls can be tricky - I have had weird things happen when I sat on one !