There's a function of Workbench
Do you have a POD device (xt, X3, or HD) with the Variax in? If so, then just connect your 600 to it with a Cat5, Cat5e, or Cat6 cable and the POD to your computer with a USB cable. The 600 and 300 have the same electronics and both work with Workbench.
The option that TheRealZap (with his awesome K-9 avatar) is telling you to look for is a global option that affects individual string volume. For some reason (albeit rare) the previous owner may have zeroed out that string, so even replacing the piezo would yield you the same result.