What is the optimal setting for the tweeter controls on the back of the cabinets when using the tweetertone on the amp?
There is no optimal or "best" position really.
You set the speakers to the max level you want to control with the knob. This means that if you want the tweetertone knob max to correspond to the max on the speaker, set the speaker knob to max. But you can also set the speaker knob to 12:00 and that will be your max from the tweetertone knob - it is all a matter of preference.