I remember when I was an audiophile newbie, I think in the late 90's, there was a buzz about a "state of the art" CD player, Ultech UCD 100, a unit that was rated in Stereophile recommended components. I bought it from a high end store in Chicago, who shipped it to me on Long Island, ny. At first listen, it sounded terrible, like a 99$ Technics or Pioneer. There was no bass, awful midrange, and very dull high end. It sounded very unengaging. Granted, at the time it was only 16 but processing, but it did have the HDCD chip and dual Burr Brown dacs. I called and complained. Before I could even describe the sound the salesman beat me to it: describing to me how it probably sounded "out of the box". He told me to put in a CD on repeat and leave it for a couple of days as I went to work. Within about 70 hours of constant play it sounded like a different player; great soundstage, much more bass slam, very clear and comfortable high end, just so much more engaging. I still have the CD player today, but use its coax output to a dac. I've never really had to break in anything like that since. But that early experience sure made me respect the concept of break-in.