First, make sure this is your bass. Then go to the pawn shop and tell them that this is a bass that was stolen from you, show them the serial number and tell them you will be reporting this to the cops. The pawn shop must then pull it off of display and hold it -they cannot sell it- until the issue is resolved. However, and again, if you have solid proof, the most practical thing is to tell the pawn shop you will reimburse them for the amount they paid for the bass. If it gets tied up in the police report, it could take weeks if not months to get it back, and the cops will then put the pawn shop on their "buyers of stolen property" list, which the pawn shop does not want. And the pawn shop will lose any money they have tied up in it, and have to turn over the records of who pawned it, and anything else this possible thief may have pawned. So it would be better for everyone to just give the pawn shop the cost of the bass, walk out with it and not file a report. If the pawn shop tries to blow you off, don't get mad or call names, be very cool and matter of fact, business like, and just tell them what I put in bold type.
PS -- I see you have matched the serial numbers. Proof in hand, (and give them copies not the original) Head to the pawn shop right now, then to the cops if they won't sell you the bass for what they paid for it.