The Nagra VPS doesn't have a volume control. So it's not really desgined to drive a amp directly. (But like any source, you could do this, if the gain matched your amp just right and you never wanted to change volume)
The Nagra PLP is a full function preamp with nearly the same phono stage built in.
eco_audio:
Not sure why you're asking about xlr inputs. Just fyi, I went from a all BAT system, vkp10se superpak, 51se, 150se. I was nervous about going with any type of single ended component. (The Nagra does have xlr outs, but it is not a balanced design). I was using a din to xlr tonearm cable and the BAT phono sounded better balanced in and out. The VPS single ended is that good. I took NO steps backwards in running my phono stage now single ended into my BAT balanced pre and amp...