In my tests, the NAD M3 is a really unique product - I have not heard another amp like it. It has a very warm and organic sound. The high end has absolutely no etchiness at all. The NAD M3 can sound fantastic even with modest sources. It has a natural musicality and three dimensionality that allows your ears to relax and the music to just flow.
The Krell 400xi is extremely resolving and has odd harmonics which you can hear and gives it a "dark" character. Very large soundstage. I would not describe this product as musical but if you have a very musical high-end source then the 400xi can sound excellent due to its resolving nature. But it will be a disaster listening to anything but the best sources on it, in my opinion. You will either love or hate the 400xi, likely highly dependent on what is feeding it.
They are very, very different products.
I have not listened to the Marantz PM11.
The Krell 400xi is extremely resolving and has odd harmonics which you can hear and gives it a "dark" character. Very large soundstage. I would not describe this product as musical but if you have a very musical high-end source then the 400xi can sound excellent due to its resolving nature. But it will be a disaster listening to anything but the best sources on it, in my opinion. You will either love or hate the 400xi, likely highly dependent on what is feeding it.
They are very, very different products.
I have not listened to the Marantz PM11.