Haven't had any experience with the Nagra amp(s), but I did do a significant demo with the Nagra PLP preamp. It's a beautiful piece and remarkably small, "jewel-like" is an apt description - BUT, I must say that I was not impressed, especially for the money. With my equipment (all tube phono stage and amplifiers), the Nagra acquitted itself well in the soundstage and imaging department, but tonally, I thought it was very "white" sounding and "threadbare" in that it did not capture instrumental timbres. Quite a surprise for a tube preamp. My phono stage through it's line stage was much better than using the Nagra's phono stage, at least so I thought. Maybe different tubes would allow the Nagra to produce the fullness, complexity and character that music has, but Nagra's stock demo unit did not do it for me.
