Yamaha DTXpress/DTXplorer/DTXtreme

It is almost impossible to diagnose what is happening without seeing
some signals.  However, I am suspicious that you have got it working
for the wrong reasons.  While I don't know what is in the game box, I
do know what the KP65 signals are like and if it doesn't work with R1
in the circuit then there is something wrong.  I think you are
possibly coupling the piezo signal directly into the game box input
through the transistor/diode, rather than using the transistor as a
switch.  

Keith.