I'm having difficulty determining the correct phase setting for my sub. Is it possible (advisable?) to determine the ideal phase setting by comparing FR plots?
Yes, most certainly. Clearly the 180 setting is more in phase with the rest of your speakers since you are getting better output at almost all your frequencies. If you have adjustable phase instead of a switch, you might want to plot 30,60,90,120 and 150 degrees to see if you can do even better.