With my FiiO m11, iPhone and iPad it works just fine. This is so bizarre
You said that your other headphones do not do this when plugged into the THX 798. What is the impedance on the other headphones? Here is why I ask... my experience with the Chord Mojo: I was listening via the Samsung S9 -> Mojo at work for months on my HD-600, HD-650, or HD-660S. These are all 300 or 150 ohm impedance headphones. Then I plugged in the Audioquest Nighthawk (25 ohm impedance). All of a sudden, a buzzing background noise became evident. I posted and asked on the Chord Mojo forum. Turned out (in my case), that this was due to RFI that was audible only on low impedance headphones. With the Sennheisers it was not there - period. In my case, because of the Mojo specifics, this was cured / is cured by turning off 3G.
In your case, I can only venture to guess that the THX 798 is the problem causing the noise but it is only audible with the low impedance MX4s. Do you have another low impedance headphone to plug in? And even then, could the big planar technology (Tesla 1.5 magnet) in the MX4 do this when coupled with that particular amp? Just providing some ideas. Maybe Audeze can provide an opinion. I see that they are viewing this forum right now.