As per Kiwibird's advice, I've pretty much started from scratch although, to be honest, when we first brought him home we were far ahead of where we are now, so we've digressed well beyond that point.
While it's possible that he was affected by Echo's disappearance, I doubt it. He never really even acknowledged her existence and she, in turn, patently ignored Sammy. He threatens the dogs (well, now, dog...) when they get closer than he likes, and the dog is smart enough to know it's time to wander away.
What we have noticed is that the first instance of his "grudges" occurred the previous time we went to Mexico...and a few days after we got our CAG "Reno". Sammy's behavior changed at that point, where he simply ceased talking, content to just jibber jabber when the mood took him (Reno is probably the most accomplished talker of any bird I've ever heard of). It's like Sammy decided "Why keep dogs, and bark myself?"
On the other hand, prior to us going away this year, Sammy was happy sitting on me on one end of the couch, while Reno roamed around on the other end with my wife. There may be jealousy there, but I'm extremely careful to not give attention to any of the other birds while Sammy can see it.
As far as we've been able to determine, he's about 12 years old. We were told he was a confirmed male when we rescued him, but I have no documentation to confirm this.
My plan, at this point, is to keep feeding him treats etc. to try to rebuild the trust (as much mine as his!), walk away when he's aggressive, and wait for him to get over his grudge. My hope is that, sooner or later, he'll start longing for his cuddles and scritches again, and decide to be the sociable boy he used to be. His orneriness is kind of adding injury to insult, after losing my little girl, and I have to try to remain patient.