I don't think there is a neutral state although I'm going to have to go back to Barrett and Lembke to think about this in light of their research. (Sent thoughts, Godfrey ?)
Practically speaking I don't think there's a neutral state.
Neurologically speaking I'm not qualified to answer that. But of course I'm happy to toss out an opinion.
The affective circumplex and the teeter totter are both conceptual models or analogies and therefore it could be assumed that they don't fully represent the biological processes at work. They seem to imply a neutral state at 0,0 or at perfectly level, respectively. But it could be that these implied states are a failure of the analogies, or that they are so infinitesimal as to be meaningless.