I don’t really think cans were ever an intended use of this player. On most units of this type, there’s a limit to how much power the DSP can handle - that’s why earbuds are ubiquitous in the embedded MP3 player industry. If you want a good example, take a look at the developer discussions for RockBox on the Sansa e200.
I’ve even seen a number of players actually blow out their DSPs over time when driving an overly large pair of phones; it’s an industry-wide design issue. Bear in mind, also, that cans are typically meant with higher-powered systems in mind. I wouldn’t plug my earbuds into the big receiver in my living room, either.
Might I suggest you get/build a small amp to use with the cans? I’ve heard great things about the “altoids tin” amplifier that you can find designs for all over the internets, and I think it’s a great idea in general. There’s only so much that the miniscule amp in an embedded player can do given size/power/cost restraints, so IMO it makes perfect sense to step the signal up if you need serious output.
