In some ways, alot of this depends on the definition of "emotion". >.>

Because it is perfectly possible to rewire all the meatspace humans in Culture to skirt around whatever specific emotion you want. The humans won't even notice... since you also warp their definition accordingly.

Analogy:
Translate a grid by +2 and it shifts. Everything on it stays the same shape, but they're all +2 to the right.
Similarly, rewire the humans to have something-not-quite-fun in their brains but to them is still "fun".


Essentially, Culture re-engineering people is transhumanism taken to its logical limits. Which, btw, assumes that people don't have souls which is why the Culture can just engineer intelligences willy-nilly.

If you think this implies all the Culture citizens don't have souls, well... I guess they're all warp-null then?