my guess is bluetooth transmitter with bluetooth headphones?
Bluetooth headphones transmitting using a lossy codec completely nullify any benefit of using a "pretty awesome" old Denon amp...
Why headphone jack? That is a amplified output, a bluetooth transmitter would require line level input. You'd need to use l/r pre-out, or record out.
Bluetooth would probably require to downsample lossless to lossy, so audio quality will be degraded.
OK, I'll make it a little simpler. Bluetooth audio streaming makes the sound quality worse. What you are trying to achieve doesn't make sense.
Are you assuming that all wireless headphones are bluetooth? There are good quality wireless headphones using different technology.