Hi Max,
I can't quite follow your post, but let me add a little to what John wrote.
While it is normal and actually recommended to connect the negative outputs of the color signals to pin 25 (ground), it is not normal and not recommended to connect the negative X, Y or Z output to ground. This should not destroy anything because the op amps have current limiters and also the output system has protection resistors, but it certainly will not do you any good...
It is possible to get the equivalent of -10V to +10V from the X and Y signals and the way you do this is by using both the negative and positive X and Y signals. Each of these (negative and positive) swings from -5V to +5V, so if you use both in a differential manor (as we recommend), then you will effectively get -10V to +10V.
Bill