- which don't describe an action so much as describe a state of being
- which don't so much describe an action as describe a state of being
Are both constructions grammatically correct? I have no clue.
Both are correct, but "describe a" becomes redundant in the second part of the phrase (in both) because you've already stated that the verb is describing, regardless of whether it's an action or a state of being.
It would be better written as "which don't so much describe an action as a state of being", etc.