The voltage reading on an SLA will NOT tell you if it's fully charged by just putting the meter across the terminals. DC voltage must be measured under load for a somewhat accurate reading. With that I would hook up the deck, pre, whatever is being run and run it...THEN check voltage. As was stated if it's a 12volt somewhere around 13 volts should be fully charged.
Again...check voltage under load cause it might look fine when it's not powering anything but then you get the show and plug in your pre...uh oh!