Do those headphones work when plugged into anything else with a headphone socket?
I mean are you sure the headphones themselves are fine.
More likely you need to set the headphone output level manually, check the manual....
If they work on your PC and i-pad, then clearly they would work on your Denon (if there is actually an output on its socket).
(By the way, recheck your measurements:
On your TRS plug, going from the pointed end you've got Tip, Ring and Sleeve.
Your 2 resistance probes go on tip and sleeve to measure left - and ring and sleeve to measure right driver - each a bit less than 2.1 k ohms now surely?)