I recently did research on this as I have the speakers myself. Griffin Technologies made a product called the iFire (the onlyproduct ever to allow other devices to use them). The iFire used to be $30.00 but was discontinued when the tech got too old to be relevant. You can still buy one on eBay or Amazon but the cost is at best $220.00 USD. I ended up connecting my old iMac G4 to them and using an app called Airfoilto stream music from my other devices to the iMac via a LAN wireless connection. Note: My Pro Speakers are those with a proprietary mini-jack that will not work with any Intel iMac. The computer I connected it to is one of the few models able to run these speakers.
P.S. Loving the Apple Speaker sound! They are the best speakers I've ever owned, and mine are 16 YEARS OLD!
-D