I have seen some SSA from GSDs but it has been few and far between for me to see it with other dogs in the house. Maybe I've been super lucky to see GSDs that aren't SSA. But Judge plays with other intact males and accepts any dog I bring in my house as well as never challenges Hobie...maybe he is an oddity but I thought he was the norm for a nice stable GSD. GSDs can be reactive to other dogs in my experience but I don't think it is that much more than other breeds. IDK. Most the breeders I listed are in WA and they have a variety of lines so could very easily give you some hands on experience with different lines. Von Grunheide in Snohomish is another great breeder, Suzanne is a K9 officer and knows her stuff!