Check that your pickup height is where it should be. Beyond that, I would think the passive/flat EQ active output of that particular bass is lower than you're used to. For example, almost every P-Bass I've ever played has had a higher output level than almost any active bass I've played with the EQ flat. Different basses=different output levels. As long as it's sufficient that you can turn your amp up and achieve the volume you're looking for, it's probably just the way those basses are. I've played a couple Sires, and didn't find them to be low-output, so I suspect your pickups might be a little low.
