Up until recently, when I opened a MIDI file it automatically connected all channels to TTS-1. This seems to have stopped working. Is there a setting somewhere that's changed and I am missing?

It works for me - I think it does or doesn't happen, depending on something in Preferences.  I am away from my rig at the moment, so I am a bit hazy on what the setting is - something about a default synth or mapping or something like that.  If nobody else chimes in, I will look it up when I get back home.
 
Bob Bone
 

Make sure there are no outputs selected in Preferences > MIDI > Devices

Thanks scook - that worked a treat. I had changed that setting when I got my new Scarlett interface to check all the connections and had completely forgotten about it.