Can't comment on the pairing with the NAD as I have never listened to it but I think you really want to listen to both before making a choice.
Only earlier today my wife and I auditioned the Concept 20, the Zensor 3 as well as the 2020i and Zensor 1 all connected to a Marantz MCR610.
Both my wife and I agreed that we liked the Concept 20 more than the 2020i and the Zensor 3 more than the Zensor 1 but the Zensor 1 was actually extremely close to the Zensor 3.
Listening to Adele, Rolling in the Deep, the Zensor 3 sounded best to us, followed by the Zensor 1, then Concept 20, then 2020i.
Listening to classical music (Mars from Holst) however my wife thought the Concept 20 actually sounded better than the Zensor 3 whilst I remained prefering the Zensor 3 and Zensor 1 over the Concept 20.
In the end we actually decided to get the Zensor 1 over the Zensor 3 due to the size of the cabinets - this is for our conservatory which is not very big (in our living room we have Ruark Templar 2 coupled to Marantz KI Signature PM-66SE and CD-63mk2 which are an amazing combination).
Don't get me wrong, the Zensor 3 do sound better than the Zensor 1 but actually, the Zensor 1 sound very, very good, too, just not quite as good, however when space is an issue as it is in our conservatory and given this is only a second system rather than the main hifi in the house we decided to settle on the Zensor 1. I personally would have chosen the Zensor 1 over the Concept 20 even.
Something interesting the helpful person at Richer Sounds said is that the 2020i/Concept 20 get a very mixed response from customers - some love them and some don't. He said both have a particular sound representation and some people like that and some don't. My wife and I appear to be in the don't love them category. We both felt they just didn't feel right - sorry that is not very helpful but it is difficult to describe. Perhaps the best way to describe it would be to say that the Zensor 1/3 sounded a bit warmer than the Concept 20/2020i and both my wife and I liked that about them.
The most important point is that both the Zensor 3 and Concept 20 sound very good without any noticable distortion either at the treble nor at the bass end, and I think it is entirely down to personal preference which "type" of sound representation you prefer so you really should not buy either without trying them with the particular system you want to hook them up to. I am certainly very happy that we made the effort to listen to all four as it made the choice much easier!
Hope this helps.
Best regards,
Anton