The material Tolex, is very difficult to remove, because it is attached with industrial contact cement.
Even solvent will loosen the material, and you can peel it off, but the cement is still stuck to the particle board.
Sanding the contact cement off is slow, and difficult, the cement clogs and sticks to the sandpaper.
It's better to peel off the old tolex with solvent, and glue new tolex on. You could also use a heat gun to soften the contact cement...
It's possible to peel the tolex as above and glue veneer, like Formica, and then trim the edges with a special router wheel.
However not practical...
Or find some wood grain monitors that you can install the JBL components into...it would be a lot easier.