The Monoprice assumes your computer has all the necessary audio codecs supported for whatever content you are playing. It probably does, but just make sure. If not, you'll need the decoders in the external device.
Yes, AVRs can be cheaper than prepros, but your first post said: >>I was thinking of using an Anthem MRX300 and just using the line level outputs (because the Anthem just sounds good)<< so it seemed you might like a good sounding, non-AVR solution. For example, the new Outlaw 975 is $550 and sounds good, so it does not have to be an expensive proposition to use a prepro.
Deadwood Atmos theater
AV7702 Atmos 7.4.4, SSP-800 PLIIx 7.4
Aerial Acoustics 7B/CC3B fronts, B&W CWM8180 surrounds, Tannoy Di6 DC heights, Hsu ULS-15 subs