ATI 4000

They make great amps.  They also are the OEM for several big names in audio.  Good stuff

Have an 1502 also that goes into the stack for 6 months of the year in rotation. Have had it for a number of years and been happy with its sound and durability and power. It is a different experience from my McIntosh MC250, less warm, more guts and good extension. Sometimes sounds a little bright depending on the speakers I am using but I have kept it which says alot.

Much thanx.

I have had 2 of the ATI 1502's and they are fast, full ranged and I thought sounded wonderful.  I played with some B&W bookshelf speakers and then with some Magnepan 3.5's, Linn Kan's and some JBL 4312's.  I have 2 son's and both were given a choice between a Dynaco ST-70 and the ATI and they both picked the ATI.  So, I no longer have an ATI. 

You are going from one class of solid state to another solid state of a similar classification, so not too sure the ATI will be an appreciable difference of sound.  I have not listened to the NAD amp. 

All I can tell you is that the ATI amps have never disappointed me.  Not too sure about their direction into class D, but that's a different story as the 4000 is a A/B. 

I don't know if you have an opportunity to have some listening time with the ATI prior to pulling the trigger?  Ideally to set the amp up with your speakers in the listening room.  Otherwise, take your amp to where the seller is and hook up to their speakers.  Bear in mind though, different speakers and different space can be a totally different sound.