Two weeks ago I would have agreed with you. I used to roll my eyes at the idea of SS burn-in. However I noticed a substantial change when comparing my iPhone (old source) compared to my new X5 over the course of 2 weeks. Originally the X5 had less bass than my eq'ed iPhone, now it has more. That is not getting used to the sound. It is a relative A/B difference that changes over time.
I understand your skepticism. I was the same.
I can second that, using my HD650 and E12 with my iPod and my new E5 the iPod has mush more bass. Also the treble on the X5 seems harsh.