Are you sure your front speakers are 8 ohms? Almost all car audio speakers are 4 ohms.

The impedence of the speaker is measured in ohms. With amplifiers, you can hook up speakers that have HIGHER impedence and not cause damage. What you don't want to do is hook up speakers that are LOWER impedence than the amp is rated for.

For example, if your amp was truly only rated for 8 ohms and you hooked up 4 ohm speakers, you would risk overheating the circuitry and frying it.

I have a feeling your deck is probably really designed to run at 4 ohms, so 6 ohms speakers would be NO problem whatsoever.