When one opens a midi file in Sonar these days, it auto-opens the TTS-1 GM vst...is it possible to change this default to say, Sampletank 3? 
 
thanks

No, but you can execute the replace synth menu option from any track.

I don't think it is possible to change this. I don't use TTS-1 for GM files. I have a Roland Sound Canvas that is set as the default. How this works is one can set a port as default for MIDI out. Because TTS-1 is a soft synth and has to be loaded before it is seen as a port I believe this is done internally and not subject to change except if one has hardware with a physical port. I would ask CW about this to be sure.