Re: Decompression technique
It does sound like the pickup has a very high output. Piezos produce their own current so I'd imagine it can be used passively - unless the preamp is doing all the work. Just try taking the battery out. Is there a discrete setting for the preamp independent of the volume output or is the preamp the only volume adjustment? To lower the output would be quite simple if you don't mind getting the soldering iron out! Just wire in a variable resistor to trim the output or a static resistor if you know the value you want.
It could also be string proximity to the pickup. Is the pickup housed in a way that you can adjust it? It would usually be fairly trivial just to lower the pickup further from the strings.