Discovered two new things today:
1. Changing out the center dash speaker makes everything come alive. I was a bit underwhelmed with changing out the corner dash speakers on an HK system to the Infinity 3032s but I just changed the center dash speaker today. That (along with the corners already done) makes more difference than stock to just the corner upgrade. My test drive turned into almost an hour as I just couldn't stop smiling at how amazing it sounded. If you've only done the corners, I highly recommend you upgrade the center - if you have one.
2. Before I put everything back together (ANC/polarity check) I ran the system and played with surround on/off. Much to my surprise the center dash speaker plays even with surround turned off. Based on this, I don't think turning surround off = true stereo. I think it's just another DSP mode. I didn't listen with surround off much but I think upgrading your center speaker makes non-surround sound better too.
I just replaced the third speaker, and yes it makes a real difference over just replacing the corners. Much easier than the corners too. I can't express how happy I am to hear cymbals again along with all the other high end sounds missing from the stock speakers.