The guy we (littleindiangirl and I) got our ASF's from, his girlfriend spent time hand taming them everyday by socializing with them. Don't know if it makes a difference.
Another thing to think about, like you were saying genetic aggression, if you hold back female domestic rats that are nervous and very skiddish, more often than not their offspring are also very nervous and skiddish, being very hard to work with and get them over their initial fear of us.
We have seen this happen for a couple of generations, nervous mother, gives nervous daughter, and nervous daughter to nervous offspring.
It is possible for us to choose the most outgoing rat from this line, and we have done so, by pairing up with a really mellow male there are bound to be easier going rats. I've gotten both results in our colony, so it seems to me you can breed for less uptight rats if the effort is put in without breaking quarantine in an established colony.