NAD T 755 A/V receiver --> Go for it.
The NAD T 755 is a very nice receiver, and if you can get it for $600, it is a very nice price. Your choice is a nice one indeed. I'll go with that one.
That will give you a very good sound for 2-channel stereo listening and also to watch movies in 5.1-channel surround sound. And it will take care of your Polk LC80i in-wall speakers, even at 4-ohm nominal impedance. They are rated at a high sensitivity of 90db, which helps a lot. So, don't sweat it.
I don't think you need to look any further, but if you want to have more options, you can always have a look also at the Marantz SR6003 ($700), or the Pioneer Elite SC-05 ($700). These two will also very well do, for your situation.
#1. NAD T 755
#2. Marantz SR6003
#3. Pioneer Elite SC-05
LOTR