As we look into upgrading our PA, it is clear that we don't (at least I don't) have the funds to buy new. So I'm looking at some used gear (Prosonus, JBL, QSC). Of course, with the speakers, it's difficult to know how hard they've been run, and as they're powered, there is risk. But how about digital mixers? As long as all features appear to function, everything slides, fades, lights ups, turns off, are there tells that would say walk away? Thanks.