The blue TB6560 boards that I have seen on eBay come with factory-installed resistors to set the maximum motor winding current at either 1.0 amp or 3.0 amps. Is your board one of the blue ones? The DIP switch settings of 50%, 75%, etc. are based on the resistor setting representing 100%.
If you have one of the boards that is wired for 3 amps maximum, and you want to run 2 amps through the stepper winding, there is no 66% DIP switch setting, so you would probably want to try it out at the 50% switch setting. The 75% setting would not be very far off from the desired value, but since it exceeds the rated current you may not want to try it.
If you later wish to increase the current, you should be able to replace the resistor value with the proper value to give you your desired motor current with the DIP switches set to the 100% value.
(I hope that makes some sense.)
As far as the microstep setting, I would start off with either full steps or half steps, just to get the motors moving, and then decide on whether and how to do microstepping. FYI, some people have reported that the 1/8-step and 1/16-step DIP switch settings on some of the boards are reversed.
You can configure Mach3 to correspond to whichever microstepping setting you choose.