How To Bluetooth

[chris.fisher]

Hi, I’ve recently connected a blue tooth transmitter

SONRU 2024 Upgraded Bluetooth Transmitter Receiver​

to my NAD 3020 Amplifier via the headphone output .using a sennheiser 3.5 mm to 6mm adapter, to connect to my Sennheiser Momentum 4 headphones. The problem I am experiencing is that the sound is distorted. If I connect the same transmitter to my Marantz CD-63 MKII KI signature headphone output the result is the same ? If I connect the BT transmitter to the CD player via optical the sound is crystal clear, but I do not have any control over the output eg. Bass treble volume.
Do I need to connect something else to make this setup work via the Amplifier?