I personally don't believe in burn-in... I attribute it to the listener getting used to the sound. I commented previously, having had the device for a day or two that the sound was sharp and quite sibilant. Well, having had it for a while, I can safely say that I no longer think the same way.
I came from a Cowon J3, and the sound from it is quite warm, and when played now against my DX50, it can sound a bit muddy and less detailed. The unfortunate problem with the DX50, and with all deeply-resolving audiophile products, is that bad mixes will sound *****. For instance, 3 Doors Down's 'Citizen - Soldier' has unfortunate wording, giving three sibilant sounds in a row. This is highly noticeable to me because the DX50 has a sharper (in a good way) high end than the J3. The J3 seems to round off these sounds - compared to the DX50 it can sound a little muddy, but it also reduces the attack of the sibilant sounds. Well mixed songs won't have any trouble, and I love the sound on most of my songs - only a few with unfortunate wording or certain singers can get fatiguing.
The vocals really shine on the DX50. With the J3, the separation isn't fantastic, but the DX50 gives it power and verve. You can hear his voice as if it were being played inside a recording booth. I'll post a full review sometime.