If you use an AMP with your powered speakers you could blow them. And it'll sound crappy. All you need is a DAC. DACs tend to sound better than soundcards. I am also using studio monitors like you are and I have an ARCAM rPAC USB DAC which costs about $250 US dollars and sounds very good. You run RCA out off the back of it to your speakers directly. It sounds very very good. I used to have the Xonar Essence STX stereo soundcard which was $200 and it didnt sound as good as the $250 rPAC. Nowhere near as good.
The rPAC also has a surprisingly decent headphone amp via the 3.5mm jack up front. And you can listen to headphones and speakers from the unit simultaneously.
http
/www.arcam.co.uk/products,rSeries,USB-DACs,rpac.htm