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...
What transmitter though,it need to go via the headphone jack.
Have a "pretty awesome " old demon amp should have nothing to do with the lossy codec what ever you call it.
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.