Ares is DA. A friend of mine who breeds corgis described Ares as "old school corgi - the type who will take it to the mat or die trying". Ares isn't going to go looking for a fight. Ares wants to be left alone. If another dog gets in his space, he'll threaten with posturing, growling, lip curls, etc. If the other dog doesn't back down, he'll bite. We've had vet bills because of it.
Tyr gets growly with other males ~ except Ares, I don't doubt that he would bite if another male got too close. Tyr isn't interested in going to look for a fight.
Often, my dogs are manageable together. I do need to be careful about having Nyx and Ares out together, because neither is willing to back down - Ares aggressively, Nyx in overbearing play. I do some crate and yard rotations, which is why I would be reluctant ot bring in another dog right now - Da or not. If a new dog couldn't get along with either of the established pairs, I would have to manage a third rotation, and that would limit my time with them too much.
My dogs are trained, Ares has a CDX. Ares, who is DA is perfectly capable of going to a dog show and competing off leash. Tyr is perfectly able to go to training classes where there are other dogs and perform off leash.
There is no need to secure them as tightly as "Fort Knox". I don't allow them to simply run loose, and when I have them out in public, I remain hyper aware of my surroundings in case someone else's dog is getting too close.
I have no doubt there will be aggressive dogs in my future. The traits I look for for their sport training tend to cause them to be prone to being DA. I very much want another dog that can work like Ares, and I fully believe that all the qualities that made him such a wonderful worker are also the qualities that make him aggressive.