level 80 bonus

i have been trying to find out if the level 80 bonus affects all classes stats or just that specific class

Do you mean the lv75 ones from Titles? Those are permanently added to the base stats of all of your characters (current and future).

If you mean the High Level Bonus Skill that you can only put points into when you are level 80, then , assuming you put points into it, it is in effect when the class is your main or sub.