Some old JBL speakers of my acquaintance use spring loaded button style posts that take at most 14 ga. wire. These adapters from Parts Express allow you to attach any banana plug. Adding a few inches of solid 18 ga. won't hurt...you should see the wire inside the speaker!
https://www.parts-express.com/Lil-Piggies-Binding-Posts-to-Lead-Wire-Adapter-Set-Red-and-B-091-1262
https://www.parts-express.com/Lil-Piggies-Binding-Posts-to-Lead-Wire-Adapter-Set-Red-and-B-091-1262