There are a lot of limitations to vBulletin's 'Advanced Search' feature. We only leave that turned on to use some search functions such as 'View All Posts By', and similar functions. If members are finding the extra search link confusing, I don't mind removing it.
To overcome that problem, we implemented a Google 'Custom Search Engine', which essentially is the same as going to Google and typing in 'site:soundforums.net' before your search terms.
Using the Google search box, there are plenty of results for "QSC GX", including a thread in the product reviews section for the QSC GX7 amplifier. See a screenshot below: