Ok, so what that means is this
Your 8 ohm speaker will not allow the amp to go as loud as possible, but will also be loud.
It's more dangerous to put a LOWER ohm speaker on a high ohm amp.
So for instance, if you connect your 8ohm speaker to the 6 ohm amp - if it was 100 watts, you may only be pushing 70 watts now.
These numbers are used for example and are not real.
Fantasy numbers below...
100 watts @ 8 ohm
200 watts @ 4 ohm
400 watts @ 2 ohm
Since you would be at the 6 ohm, maybe it would say
150 watts @ 6ohm