That's average?
I have never heard of that sub ... That doesn't mean much though. Just maybe ask around a little more.
With a room that big I would cut back to a 5.1 system that your rec'r can power more effectively for now. Then maybe down the road when you choose to upgrade, you can pick out a mean mother of a rec'r to power up the front sound stage of your choosing at that time. The speakers are the most important part of it all and those are nice sounding speakers. 7.1 over 5.1 isn't that big a deal. If you want to fill that room with sound, it's going to take something a lot meaner than those speakers but for now they will do the job of playing movies and music nicely, just not at high SPL's.
As far as rec'rs go, there are plenty of Marantz fans and unless you hear otherwise I would think that one is just fine at that price point.
Just curious but what made you choose that rec'r and that sub?