The WM08 module has Minelab's Wi Stream proprietary wireless technology and will only pair with an Equinox 600 or 800. The WM08 can offer several alternatives to using the Minelab ML80 Bluetooth headphones.
Lets say your ML80s battery runs out so you don't have a Bluetooth connection anymore (the Equinox has a Bluetooth transmitter and a separate Wi-Stream transmitter. So two completely separate, incompatible wireless systems in one detector) I can plug in a 1/8" two male plug cable into my ML80s and plug the other end into my WM08. I reset the wireless system on my Equinox, turn on the WM08, let them pair up and hook the WMO8 on my shirt or coat front pocket, stuff the headphone cable in my pocket and I am still detecting without a headphone wire connected to the Equinox. So no cables to trip over.... AND the WM08s Wi-Stream technology is much faster than Bluetooth so there is absolutely no lag. Instead of plugging in the ML80 headphones, you can plug in any 1/8" plug headphones or earbuds. The sound is great. You can also buy the Minelab 1/8 to 1/4" adapter or use some of the less expensive third party Equinox specific ones and plug in your favorite 1/4" plug metal detecting headphones.
No need to buy a new one if you have an Equinox 600. Just check on these forums. I see one for sale for less than 100 dollars all the time.