I'm about to buy a used pair M3, but how do I tell if it's M3 Turbo S vs regular M3? The serial field only has numeric handwritten serial numbers while some of the online pictures i found online of M3 Turbo S have "___ Turbo S" in the serial field. The current owner said he was told by the dealer he got the speakers from told him they're indeed M3 Turbo S.