I'm MAO-A +/+ but I'm not really sure what role this plays in all this. I think the problem is acquired not genetic.
I think the relation between serotonine and MAO A is not clear to us yet. It's probably not as straight forward as we'd like. Just like all that "warrior gene" talk, it's wanting the world to be simple. It isn't.
I think MAO A +/+ has most effect when looking at the excitatory neurotransmitters as a group. Mainly epinephrine. Serotonine not so much.
The effect is noticeable in a way a person gets excited. I "ignite" easier than another. I don't mean in anger but in enthusiasm; in alertness. In decisiveness. Typical Epinefrine-traits, I believe.
It takes me longer to wind down than other people. Once excited, these neurons keep bouncing around my synapses, it seems.
other anecdotal info: the MAO A gene is located on the X chromosome. For me, as a female, to have +/+ means my father only ever had a + and my mother has at least 1. She has the same mid night insomnia I had. My brother has too. It's some sort of cortisol-surge. Or slowly broken down epinephrine? You wake up all alert and ready to do something, anything. Your mind is sharp. But you're not alarmed or sweaty. It's only the brain that's on.
It takes 90 minutes for it to subside. (Half life time of epinefrine ought to be minutes so no idea)
Looking at my fathers family you can clearly see the "easily ignitied, keep going duracell bunny-trait" running in the family: the girls have it more so than the men but some men (my father) have it worse than the girls. They are annoyingly driven and "quicksilvery" people and are always "on". But all have different characters and different situations in which they grew up.
Anyway. TL;DR: I think MAO A is more noticable in how exitatory ones brain gets and stays then the serotonine levels.