I'm also running big Polks and first tried upgrading receivers to a Dennon 3801. The sound improved, but a year later I was buying seperates and have never looked back. Once you've got the bug, you might as well save some money by skipping the "better receiver" phase and set about building the system that you want.
The Sony will sound better with a seperate amp.
With your room and speakers, I'd buy the biggest amp that I could afford.I don't think you'll ever regret it.