I see what appears to be contradictory statements about break in. But you are all basically correct based on my experience.
Teo says if something sounds bad, don't keep it. Well good, but when you find a product that does sound good, what do you do?... Break it in to get the most out of the component.
I agree with Geoff, that very low power, like in interconnects (2 to 6 volts) is not enough. A burn in devise is needed. Don't get too wild here like I've done in the past. Be safe and practical.
Finally, an important factor is time, mentioned by the person taking notes over several months. Also the FM noise is great for burning in tubes. It only takes a couple of hours of the noise along with some music for tubes in my experience.
So go with a burn in devise, a burn in CD, and music. This will drastically reduce the time needed for full burn in and stabilizing the sound.
Regarding Class D amps: I have heard very good ones, and some that sound like something is wrong somewhere. You just need to listen to a few of them to know.