Highly recommend the East Brooklyn adaptor, with ZMF pads for the M40x’s. It’s a great “Bluetooth from TV in bed while your girlfriend is sleeping But can’t sleep without you in the room” thing
East Brooklyn Labs Bluetooth Adapter and Amplifier for Audio-Technica ATH M40x Professional Headphones from Wired to Wireless Amazon.com: East Brooklyn Labs Bluetooth Adapter and Amplifier for Audio-Technica ATH M40x Professional Headphones from Wired to Wireless : Electronics