[irox]

I've not tried 8" rounds in the back, but one important consideration for rear speakers is clearance on the top/front part. I forget which brand/model 6"x9"s I put in the rear, I did a bunch of research and thought I'd gotten a great set for the money. But they some secondary speaker/tweeker type thing in the center which protruded up slightly, which made the job of sliding the back deck/shelf on really hard. In the end I got the back shelf on, they sounded fine once installed, but I'd dread taking the back shelf off again since it might damage the center bit.

Make sure you rear speakers are flush and don't have anything sticking out/raising up in the center (or any where really) over the stock speakers.

Cheers,
Ian.