Try to find somewhere that offers the IDEXX snap 4DX test. It tests for heartworm as well as lyme, erlichia and anaplasmosis. The last 3 are all rickettsial diseases that can cause behavioral changes as well.
If medical causes are ruled out, there is a great group on yahoo groups called agbeh. It is the Aggressive behaviors in dogs group.
It is very strict due to the sensitivity of dealing with aggressive dogs, but the files and links on the group are great, and reading through the archives is invaluable.
-full disclosure: I am not the owner of the group, but I am a moderator for the group.
My experience with my unneutered male was not good when another neutered male came to stay with us for a short period. I had to keep them separated. My male disliked even this arrangement and would pee in his crate to display his disapproval of having another male in the house. We later had a female pup in the house and our male got along with her great. I aksed for suggestion on how to get them to get along and nothing worked. Someone even said that it might never work.