I may be wrong, but I think the amp section in the 5790 and 5890 are the same as the amp in the 657 and 757. The amp section in the 657 and 757 is exactly the same as the high end Yamahas (RX-V1500, 2500, etc.). I recently purchased a RX-V1400 and it is amazing. I knew it would be a good HT receiver, but my real suprise came with the musical aspect of the unit. It is by far the most natural and true receiver as far as music sound processing. The HT portion is exactly as I expected (even a little better actually). I have not heard any NAD receivers, but I know they have a good rep which usually goes a long way.
However, I can tell you that if you go with a Yammie you will not be disappointed. Just a suggestion, unless you absolutely are set on getting the 5790, I would look at the 657 and 757 or even the RX-V1500 (or 1400 if you can find one). They are all very close in specs (the 657 and 757 have no multiroom and no THX, but have the same amp and sound processing as the 1500). Just my $.02, I am sure you will be happy with either receiver.