There's no mechanical or electronic adjustment that can be done internally to the 4VF to adjust the trim. I use the 4VF buddy box with my 9C and in the training menu, I can set the channels to "function" which automatically changes the trim of the buddy box (electronically) when any trim changes are made to the master (9C). I think you need to make mechanical adjustments to your control surface hook-up until you can find a correlation between your 6XAS and 4VF. The trims on both may not be center though. Make sure you have enough room for in-flight trimming especially on the elevator. Good luck !