I agreee with Bandphan concentrate on speaker upgrade.
upgrade your speakers first
Spending money on a new avr of that caliber with those speakers is a crying shame
Bandphan makes an excellent recommendation. The front speakers are most important. The center speaker should match to maintain timbre ( seamless front sound). The recommended matching speaker or a third speaker like the fronts can be used as center. Center is important if you listen to TV or movies because it carries most of the voice dialog.
The surrounds are for enhancement and don't have to match the fronts. Even thought really, really true hard-core audophiles insist that every speaker is the same
. You could reuse your current Polk Audio RM6200 speakers as surrounds; that is what I would suggest.
The subwoofer also doesn't have to match. SVS and Hsu are good brands and a subwoofer can be added later.
Going back to the front speakers, if possible, audition them and audition them with the type of music that you normally listen too in a sound room that most closely depicts your actual setup. Think of how much $$$ you want to spend and we can provide some great recommendations. There are a lot of good sounding speakers, but you are the final judge and that is why audtioning is important.
Good Luck,
MidCow2