there are several options: amp - serves to unload the signal before sent to headphones. also, an impedence adapter. the amp is more expensive but a better option as it does not change the character of the headphones, only serves to show the dap as its best.
the denon will not sound bad, but they are not well controlled by the denon. when i said that the bass gets blurry, for my music it was abysmal for some songs. the denon are good but the bass is not as controlled as some phones. they have run into control issues with d2. with the ipod touch, they never resonated to the point of losing all resolution and becoming a thump like you hear in poorly done car sound systems.
denon are good phones, but they are a special make requiring a good system to really sound good. the d2 is not bad nor anywhere near it. it is just normal or perhaps even above normal. in certain musics it does better than my nano 3g but overall very similar performance. it does never boom with my nano though.
the denon does not show off the weaknesses of a system the same way that er4s or p does - it merely will not sound its best without a really nice system. it is wonderful through ipod - lod - amp. good luck.
sorry, sony and shuffle have a softer edge to the bass and resolution - after hearing both of those cheaper solutions, i cannot forgive my d2
