In my design, I pass the piezo signals through a voltage divider prior to running the signal through the first op amp for level holding; this serves a double purpose of bringing the (potentially tens of volts) signal down closer to 3.3v logic levels. Originally I was using a 1k in series with a 200 ohm resistor to ground for the divider, for about 1/6 the signal. In my breadboard testing that proved to be correct, but in real life (with better drum pads) the signal was reduced too much, so the signal to noise ratio of the input was horrible. I decided to change the 200 ohm resistors out for another 1k, and tests showed that this would work much better. However the problem remained of how to actually do it. Trying to desolder with my iron made a mess of the beautiful board when just doing one resistor (of 12 total), so I knew I needed to do something else.

I was reluctant to use hot air to do the rework; my hot air 'station' is a $25 Canadian Tire heat gun used for stripping paint, and the aiming precision is... poor. I needed some way to remove the resistors without melting the through hole pins and damaging the other components.

The solution was to wrap most of the board in tin foil, leaving only a slit still visible. I was nervous when doing it, but it turned out wonderful. Below is a picture (after the old resistors had been removed, and I had replace them with new 1k ones with solder paste, but before I reflowed the solder):

Now that the voltage divider passes through higher signals, the board is much more accurate. You can see the video below for a demonstration. The camera's microphone input is hooked up directly to the line out of Drum Master, so you can get a good idea of the latency (essentially nil). The microphone that I was talking into was plugged into the line in on Drum Master, but since plugging a mic into line in doesn't give the best volume, it is a bit difficult to hear me talking... turn on subtitles to help understand what I am saying. Please excuse my horrible drumming skills, it has been many years since I have played (and I was pretty bad back then, too!) ;-)