Get a 6.1 system for starters. You're missing out. You can't hear things like the arrows whizzing over your head in the first scene of Gladiator without that. Even if you can't get the side speakers yet give yourself the room to upgrade.

Then move those main speakers to the corners of the room. The sub can move to the middle. And your center speaker needs to move below your monitor- it can't sound right being up there. Try a bigger rug and a denser window covering- I suspect that room sounds pretty hard.

The receiver is tertiary. I own a Denon but a Marantz, an Onkyo or a Pioneer etc. are all pretty comparable to it and each other. What was said about Cambridge is true- they make remarkably nice components especially for the price. But a few changes to your room and the placement of your speakers can make a huge difference and cost you little or nothing.