Yes, either the PMS GTN750 or the TDS GTN750xi can be set as the default by adding a line of code to the .FLT files as outlined in THIS POST shared by user Cygnifick which was posted quite a ways back in this thread.
Be advised though, the string of code in that post only works for making the PMS GTN750 the default. To make the TDS unit the default, add these lines to the LocalVars.0 section of the PC-12 FLT files instead:
AS530_HIDDEN=1
GTN750XI_INT_HIDDEN=0
That worked for me. To cover all boot scenarios, I’ve added that to all my PC-12 FLT files (i.e. approach, apron, climb, cruise, final, hangar, runway, & taxi) and it now loads the TDS by default.
Here’s how it should look when you’re done:
Remember, don’t alter any other lines of code already in the LocalVars.0 section (usually there’s only 1 or 2 lines, depending on the type of FLT file), just leave them be. Just ADD the two pertinent lines at the end (as illustrated above) and you should be good to go.