As Arcam_boy says you don’t necessarily require the NAD Pre amp - if you did want to integrate it then the NAD M12 has the option to set the Analogue Stereo Input to a ‘fixed’ level and use it in Home Theatre Bypass mode.
Keep you AV Sources connected to your AVR and connect the Front LR Pre Outs of the AVR to the Stereo Input on the Pre Amp, then connect the Pre amp Outs to the Power Amp Input and finally connect your Front LR Speakers to the Power Amp.
Connect you Music Sources direct to the NAD Pre amp.
You may want to have a think about how and where you connect your Sub too!
Joe