Amp for B&W 706 s2

NAD might sound a little better, maybe, but reliability and factory support has degraded. The  Yamaha will be (probably) trouble-free for years.

Your choice.

Enjoy your new speakers! I bet they are amazing. My B&W 803 D3s sound amazing to me too.

Krell, Arcam or Classe amps

Well you are doing the right thing by replacing the AVR with a 2-channel integrated. You have a nice pair of speakers 

The Yamaha is a more complete package regarding streaming services built in making it a more all in one unit. If purchasing from a dealer you should be provided with some support for a new unit.

The NAD is probably a more robust amplifier, more powerful. I believe the original price of this unit was about 2X the Yamaha. If this unit has been working flawlessly for the seller and in good condition it will likely last for years. For my tastes believe I would prefer the NAD however not have heard it with your speakers. I would also prefer the streamer/DAC to be a separate unit. My tastes may not be yours.

All that said I can't make the decision for you, only provide some thoughts. 

The choice may come down to feature 

Thank you all for the answers. The Nad unit is in amazing condition (i have took other parts of this friend before and worked great). I think i will for Nad. Ok..yamaha has everything i need in 2022 but i will add an external dac/streamer. Also, i cant reach higher prices and of course i know there are better amps but i want to do the best choice for around 700-800€. Nad is still in stores here in Greece (nearly 1400€). Another question...i have a velodyne subwoofer cht 10r. 1) can i connect in pre out on Nad??? 2) Will the sub volume change when i turn up/down the amp (Nad) volume??

Look online for the manual for the NAD. I have owned the 326 and 356BEE. Both have a variable pre output for a subwoofer.