Those Monitor Audio Silver S2 speakers are nice - they're around $1000 new if I'm not mistaken - but spending $2000 to $2500 on an integrated amp and less than half that amount on the speakers seems to the reverse of what you'd want to do. I've always had better success putting more money into the speakers - that component (along with the room) has the biggest effect on the sound you're hearing. If you're not hearing the details you want, you should start with the speakers, not the integrated amp, especially since the amp you already have is a nice NAD unit.
I know this is not what you want to hear, since you recently bought the speakers, but if you spend $2500 on an integrated amp and keep the same speakers you have, you're pretty much going to hear the same sound you're hearing now - no matter how good the amp is, the speakers can only do what they can do.
My opinion is you're better of taking the $2500 you have in your budget and buying the best speakers you can possibly afford. You'll have a better chance of success than throwing a $2500 amp at a $1000 pair of speakers.
