the problem is with "stuff", not with "all... are".
"Stuff" can't never "are", because it's a mass noun.
Foodstuffs are a different matter, on account of pluralization making it into a sort noun. Like this wine is nice, those wines are horrible - wine is a mass noun in singular, but a sort noun in the plural. With a classifier the mass noun word becomes quantifiable without turning it into a sort noun : "These bottles of wine are spoiled"... in this case it could all be the same sort of wine, or it could be different sorts or any combination of the two - it's a singular object noun (a different type than the other two).
There are also set nouns which denote a set of entities, these often stay singular even with numeric quantification (in languages where set nouns are the norm, most nouns behave like english "sheep" - one sheep, two sheep, etc.
More accurately they can be seen as having a finite, definite set of entities to choose from and the numeric quantifier works similarly to the english sentences "three of sheep" or "three of the (relevant) population of X".
Sorry if I got lectury, I just thought I'd share some strange trivia.
Have I mentioned that there are languages where numerals (and colors) are handled like verbs?
"The sheep was threeing on the lawn, eating"
"My uncle was the sheep blacking of the family" :D