Barry,
First off allow me to state that I'm a bit of a Nuprime cheerleader, so take my thoughts with this knowledge. I would not expect the Arcam amp to be better sounding than your AMG STA - quite the opposite. The AMG STA should be a much better amp than what is inside the Arcam. The DAC-9 is what I think of as an entry level DAC. It's not bad sounding, but there are other Nuprime DACs that run circles around the DAC-9 in my opinion. I had the DAC-8 and the DAC-10 in my system several years ago, but I sold them as I upgraded the rest of my system. I have a DAC-9x in a second system, and I think it's great for the money. It sounds really nice driving my EVO STA amplifier, and it sounds nearly as nice driving my tube amps. I think it pairs better with the Nuprime amp - perhaps there's some synergy there?
While I have not heard your speakers, I'm inclined to agree with what AllanS said. Upgrading your speakers could well make a big improvement in your system. Secondly, adding a second AMG STA and/or upgrading to a better Nuprime DAC afterward would yield benefits.
Now I'll ask the questions I should have begun my reply with:
What size is your room?
Where are your speakers in this room?
Have you used any sound treatments on the walls?
Is your floor carpeted, bare wood, other?
What kind of music do you prefer?
Michael