Diagnosing sound issues

Hi, all. This question is actually not about guitar, but about my electric cello. Hoping someone can offer some insight.

I have a 5-string Ned Steinberger electric cello. This instrument has piezo pickups in the bridge with a built-in preamp powered by 2 9v batteries. Basically, whenever the input signal gets above a certain level, the output signal becomes very distorted and flabby sounding. This happens when I play more than 2 strings at once, and also when I change bow directions, which causes a brief spike in the input level.

NS wants me to replace the entire "circuit board", which is really (I think) the preamp module. I've taken a look at it, and don't see any obvious faulty components; all the caps are physically intact, and I don't see any dust or damage on the board. All of the solder points appear to be intact. Of course, I realize there can be a problem with any of these components that isn't visible to the naked eye.

I guess what I'm hoping is that someone could point me in the right direction for how to troubleshoot this. At this point, I'm pretty much open to anything, considering that the alternative is to replace the preamp, which is $170 for the part alone. I can post pictures later tonight if it would be helpful.

Thanks!