Have you run into issues with installing 4ohm speakers?
I missed this thread. Quoted text is likely the most important to mind.
The amplifier uses IC's for the actual amplification, and they're rated for 8, 4, and 2 ohm loads, at various voltages. I have run the McAmp on 4-ohm mains, front and rear, and a 2-ohm subwoofer, basically halving the impedance of every driver. The only thing I noticed is some extra heat. Surprisingly enough the factory fuses stayed intact.
If you're sticking with the McAmp, keep the rear 6x9. If you change amps, change the rear woofer or you'll cook/distort the **** out of it. It's a picky, hypersensitive creature. I have no experience with Rockville yet, but I've heard that if output is your primary concern, they're good for the cost.
The Polk speakers will do you good for mains. If installed properly, they'll create enough bottom end to take over for the rear 6x9 if you switch amplifiers to, say, a 4-ch amp (can be integrated into the McIntosh head unit/factory stuff fairly easily).