Seig KX1 has a DC 500w Closed Loop Brushless DC Spindle Motor that drives the spindle with a belt.
Polarity just needs to be reversed either by physical wiring or some Mach3 settings need to be corrected.
Backup the current Mach3mill.xml (or other profile) and try to restore a past version of Mach3mill.xml.
If not please upload your current xml file to forum for others to analyze and help correct.
Amazing what can be learned from someone who knows!! Is the spindle reversible, as in the gcode??
Well according to this it only runs in the CW direction PDF manual >>
http://images.harborfreight.com/manuals/66000-66999/66051.pdf Seems like the poster needs to find out more about his machine? If its not reversible its nothing in the setup file or gcode, so it must be the wiring.