Not sure you're going to get the best out of the Promitheus with the NAD. One option to consider is the Lightspeed attenuator. This is a passive preamp using photo-optic technology. No transformers and no resistors. I just bought one and it is everything it is advertised to be.
Go to DIY.com and do a search for Lightspeed. There is a very long thread on the preamp. Nelson Pass even participates and IIRC uses the technology. The preamp is available for about $350. Single input and single output only. This will work with your NAD as it is pretty immune to impedance matching. Also, the new Pass B1 buffer is another option at $895. Again will work with the NAD.
Go to DIY.com and do a search for Lightspeed. There is a very long thread on the preamp. Nelson Pass even participates and IIRC uses the technology. The preamp is available for about $350. Single input and single output only. This will work with your NAD as it is pretty immune to impedance matching. Also, the new Pass B1 buffer is another option at $895. Again will work with the NAD.