By itself, an amp will make the HD 555 sound better by sending a more powerful signal to the phones. However, it can also magnify whatever sounds bad in the chain. So, if your laptop has a poor quality sound card - like most laptops, and if you are listening to poorly recorded or low bit-rate mp3s, then the bad sound card or the low quality music will have its bad quality amplified. This is why people here on head-fi start tail-chasing - new headphones leads to new amp which leads to new dac (in place of sound card) etc. Every time one piece in the chain gets better, the weaknesses of the other pieces are apparent.
I use an HD 595 at work which has the same drvier as the HD 555. I have tried it straight out of my laptop, with a Lisa III and with a Matrix M-Stage. Both amps make a difference. I generally like the M-Stage better because it delivers more power but both amps make a difference.
So what is the difference? With an amp the sound is fuller. There is more space between instruments. The base is more pronounced. There is more detail in the music. Think about what it does to make your music louder. People turn up the volume to put more power behind hte music and hear it more fully. The more powerful the amp, the more it puts support behind the sound and gives you the fuller sound of high volume but without the volume turned up so high.
I think the HD 595, and by extrapolation the HD 555, does decently without an amp. An inexpensive portable amp will do little more than add volume and a volume switch. I think they do pretty well with the M-Stage - which is around $250.