All amps have a "signature" sound, so depending on the amp you choose, there will be some difference in sound, as an example, Anthem gear will be a bit more forward in nature, ie, highs, and mids come through stronger than the bass notes, whereas a rotel is more laidback and will not emphasize the highs as much, Higer end amps like Theta, Bryston, Levinson will not emphasize any specific area, and are more nuetral, meaning everything is a bit more equal, but provide the reserve power and punch of the lower frequencies that the others I've mentioned will not be able to replicate. (many more, these are just quick examples)
The choice of amp WILL make some difference in the presentation, typically not noticed by everyday listeners like most of my friends, and my wife, but to me, I can notice the differences.
If you are asking this question, I would tend to think you are just looking for more power and should have many options, but in short, if you buy a seperate amp, your headroom will increase, your speakers will sound better, and you will be able to upgrade later w/o worrying about the power side of your setup, which will give you more options as you go along in this endless quest.