Yamaha DTXpress/DTXplorer/DTXtreme

anyone here have any issues with stability of settings? Having gotten 
my kick pad working ok- suddenly last night it starts double 
triggering- severely. Went to trigger settings and changed self-
reject settings and saw some improvement, but if you pick too high a 
setting you cannot trigger fast enough. I'm wondering if piezos are 
sensitive to temperature changes or if there is some inherent 
instability. All along I have had no success setting the kick pad so 
I have a semblance of dynamic range- it has acted more like a 
switch:just 1 loudness no matter how hard or soft I whack the thing. 

Since I have no manual for the kick tower- who can tell me the 
purpose of the polarity switch?

Thanks.