As if my T51 knows that it will have a little brother, it is starting to have its 'moods'. The left side can sometimes sound muffled and requires a tap near the jack to get it sound normal again. I think the T51 is biased towards the right now. The port might be faulty somehow.

Haih, meragam la pulak player aku ne.

Burn in is where you get the component to "loosen up". Get it conditioned to its working conditions, possibly unlocking its full potential. I only believe in burn-in/run-in for mechanical stuff like headphones, earphones, engines. Some say that capacitors do have a tendency to change after some use so it may perform better over a period of time of use.