Yamaha DTXpress/DTXplorer/DTXtreme

I have tried to make the circuit to work with two pads but run into 
problems. It seems to require the pad to be hit really hard in order to 
make the rim shot sound. Reducing the R1/VR1 combination did not help, 
and it caused the input impedance to drop, to the extent that it simply 
killed the sound completely.  

My guess is, becase the transistor has no bias circuit, to make it turn 
on, the piezo has to provide all the current and voltage (at least 0.5V 
B-E), and it is asking too much from the TP60 pad.

Has anyone tried to modify the circuit to make it more sensitive, maybe 
by using a FET instead of BJT? Any ideas?