There are some great products that work well on leather - I do believe the 900x are made from real leather, while the predecessor (900) had pleather ear pads. We have some leather furniture in our home. We purched some excellent leather conditioner from an equestrian shore in my area. Normally, people buy the conditioner to preserve their saddles harness, etc. that are made out of leather. In our case, the conditioner really worked to bring out the softness in the leather since the leather had dried a bit and became a bit stiff. The conditioner was the consistency of a motor oil and of an amber color. Basically, the leather was treated and allowed to sit for a few hours while the conditioner naturally worked its magic on the leather. We then went back and polished up the leather with towels and removed any excess conditioner, so it would not get on our clothes.
Prior to buying the equestrian stuff, I've used conditioners for my auto seats from Lexol since I had leather in a car. Lexol makes both a cleaner and a separate conditioner. You can find Lexol at most automotive parts stores for about $10 - $12 in the U.S. Finally, I did stumble across a "natural / home made" leather cleaner and conditioner. Basically, you combined one part of white distilled vinegar to one part of olive oil. Combine the ingredients as best that you can, then use a cloth to wipe and clean the leather surfaces. The vinegar provides the cleaning power for the leather and the olive oil offers the protection for the leather. Kind of odd, but if you search for it, you might find the recipe. For the ear pads, I'd probably combine one part vinegar and 2 parts of olive oil, since you're mainly looking to condition the pads to make them a bit softer and not really looking to clean them at this time.