You've asked about using these amps with a Nagra PLL.
I had the combination of the Ayre MX-R for about a year until I replaced the Nagra with the KX-R. The combination worked fine, and I was able to hear the advantages of Ayres over my previous Audio Research VTM 200's. I had learned to live with what I thought we congested tweeters on my WP 7's, but once the MX-R's were broken in there was a substantial improvement on the top end, particularly with female voices and solo piano. I had thought that the talk about "making poor recordings sound good" was probably marketing hype, but I do find that it's much easier to listen to recordings that used to be tough on the ears, without giving anything away in terms of detail. Maybe it's the higher power rating, or the zero feedback, but somethings different and it's better.
Replacing the Nagra with the KX-R was initially disappointing, but once broken in the soundstage really opened up, and became effortless in terms of
placing the instruments in space. The combination of the two is amazing.
My experience with the Ayre units has been extremely positive. I have not heard the Dartzeels, but given the value of the US dollar and the fact that Ayre provides excellent customer support and is a stable US company, I had no trouble deciding on the MX-Rs
MJF