I have very little experience with the SS, and what I did was YEARS ago. There is a PMDX app note on spindle speed control with a SS and a PMXD-127/107 combo. Go to the PMDX site and click on "Application Notes" along the upper left edge of the screen under "General Information".
On the PMDX side, if you have a volt meter you can measure the voltage coming out of the PMDX-107 from the Aout to Agnd terminals and also from Aref to Agnd. It also depends on how you have the 107 wired to your VFD, and how you have your VFD configured. And how you have the 107 DIP switches set. And probably a dozen other things. If the APP note doesn't help, post a picture/drawing of how the 107 is wired to the VFD and your Mach3 profile (XML file). I don't promise any help with the SS specific config, but I can look at the normal Mach3 spindle config and see if I see anything funny.
The SS plug-in was (long ago) non-intuitive, at least to me. If the normal Mach3 settings look OK and the wiring looks OK, this is probably a question better posted to the Warp9 forums.