Most air compressors used for water treatment have a pad type air filter in the air inlet. You might want to take it out and see what all it has filtered out and if it smells and clean it.
Air injected into the inlet water of the Duke should be oxidizing the iron into rust (ferric iron) in the water line and reducing the water flow plus possibly blocking up the spray heads. A dirty air filter on the compressor should reduce the psi. Blocked spray heads should too, plus your are running 50/70 psi but I'm not sure it's on the well pump.
If the submersible pump in the Duke is standing upright, you will have more stagnant/dead water in the Duke to get smelly and less if the pump is laying down, IRB can be anything from clear to black snotty/slimy coating above, at and below the water line. It can produce an odor many people would call H2S but it is slightly different IMO. So can Coliform bacteria.
If the Duke were removing all gasses, you shouldn't have any odor in the water unless the stagnant water in the Duke is causing it. I suggest checking the pressure tanks for a broken bladder. To test rock the tank side to side and see if it is heavy. You can leave a spritz of air out and see if it is misty/water vapor but if the water above a broken bladder isn't high enough you won't get mist or water out and the only way is to shut off the pump and drain the tank and then see if it is still heavy. At the same time you can check for proper air pressure; it should be 1-2 psi less than the turn the pump on switch setting. I.E. 50/70 gets 49-48 psi air with no water in the tank.