Replace those factory speakers with the new Flipper Fidelity sound system. This system consists of (2) 4" coax speakers for the backbox, and an 8" woofer for base cabinet.
This kit is compatible with the following Stern machines:
Batman 66 Premium/LE/Super LE
Aerosmith Pro/Premium/LE
Star Wars Pro/Premium/LE
Guardians Of The Galaxy Pro/Premium/LE
Iron Maiden Pro/Premium/LE
Deadpool Pro/Premium/LE
The Beatles Gold/Platinum/Diamond
The Munsters Pro/Premium/LE
Black Knight: Sword of Rage Pro/Premium/LE
Jurassic Park Pro/Premium/LE (Stern)
Elvira's House Of Horrors Premium/LE/Signature
Stranger Things Pro/Premium/LE
Teenage Mutant Ninja Turtles Pro/Premium/LE
Avengers Infinity Quest Pro/Premium/LE
As with all Flipper Fidelity systems, all installation hardware is included and nothing on your machine is harmed.
NOTE: 
DUE TO THE DEPTH OF THIS BACK BOX AND THE LOCATION OF THE POWER SUPPLY, THE BACKBOX SPEAKERS MAY NOT FIT CORRECTLY AS STOCK.  THE CARDBOARD SPACER RING MAY NEED TO BE REMOVED DURING INSTALLATION.