NAD M3/Marantz PM11

I have a ton of experience with a ton of both seperates and integrateds, especially the brands your looking into. I would get the 400xi but mated with an exceptional PC and IC set, together with a great balanced CD/SACD player. The upgraded SACD standard V3 is a giant killer, but only available/affordable for previous owners...the purity of the combo is stunning. The only single ended CD player I would consider would be the MF A5 CDP.

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.

I had an M3 as well but found it "ODD" sounding as it slid into it's hyper drive power or whatever they call it. In other words it changes character when pushed hard. It also had a relentlessly detailed upper frequency range. Funny, but I could not relax at all with the M3...go figure!