I guess I need more coffee. I haven't figured this out yet, but had a thought.
You indicated in your original post of the thread that it worked in older projects where it had already been inserted.
That being the case, why not try to create a track template from one of those old projects, by selecting a midi track that is assigned to TTS-1, and right-click on it and click on Save as Track Template, giving it some brilliant name like TTS-1, and then you might be able to simply insert that track template into a new project and end up with Sonar loading TTS-1 into the synth rack for you, and adding a midi track assigned to it.
Just a thought,
Bob Bone