Did you also reset the headphones? Owners manual page 57:
ML 80 Factory Reset
Factory Reset will return the headphones to factory presets, and will unpair all paired wireless devices.
1. Turn the headphones off.
2. Press and hold the Multi-Function button for approximately 10 seconds, until the headphones beep twice and the Status LED flashes pink.
3. Release the button. The headphones will now be in pairing mode with the LED indicator flashing blue and red.
I had a similar problem and discovered that the iphone is a bluetooth parasite. My iphone will mysteriously automatically turn on it's bluetooth whenever it is shut down and turned on again so be aware. Anyway, when I opened my new Equinox 800 and turned on the headphones playing around, they must has paired with my iphone and I never knew it. One week later I was in the field and tried to pair to my Equinox with no success. I didn't suspect the ML80 was already paired to my phone and I tried everything except the ML80 reset. Once I got home I read this post and the ML80 reset unpaired my iphone. I turned the bluetooth OFF on my iphone then turned on the equinox 800 and waited for the bluetooth to search, then I turned on the ML80s and they paired perfectly.
Wireless Headphones Won't Connect
in Minelab Equinox Forum
Posted
I had a similar problem and discovered that the iphone is a bluetooth parasite. My iphone will mysteriously automatically turn on it's bluetooth whenever it is shut down and turned on again so be aware. Anyway, when I opened my new Equinox 800 and turned on the headphones playing around, they must has paired with my iphone and I never knew it. One week later I was in the field and tried to pair to my Equinox with no success. I didn't suspect the ML80 was already paired to my phone and I tried everything except the ML80 reset. Once I got home I read this post and the ML80 reset unpaired my iphone. I turned the bluetooth OFF on my iphone then turned on the equinox 800 and waited for the bluetooth to search, then I turned on the ML80s and they paired perfectly.