The problem is the excursion factor .....that's the word I have been trying to find forever , let me explain
Excursion Factor = Driver's speed to keep up with the low frequencies , SPL is not the word , max SPL results in breakup , excursion isn't breakup , the resonating frequency of the 598 is at 60Hz at XXdb , I don't know that exact number but its very low , the 6XX is free of those distortions , the 6XX series have fabulous excursion factor just like the hd800 , 598s are just horrible , I think those should retail for $80 , Beyer DT models , Brainwavz HM5s etc just blow them out of the water in terms of excursion factor and fidelity for the similar money
Its one of those peculiarities I have observed with many open mid-fi dynamics , for the people who say they don't hear that on their 598 don't know what they are talking about unless "BURN-IN" increased their excursion factor , there is a cure though , that cure is "Output Impedance" , I have heard significant improvements from a amp with a OI of 10ohms to one with 0.1 , it will not eliminate it but surely decrease it , that's because the amp is trying to drive the voice coils with full control , more the control over the coil let's just say "refined" the membrane movement is
I have spent many months figuring this out , if you want to solve this issue throw the 598 or give it away and get a real headphone like a Beyer DT or the 6XX which are significantly superior to the 598 tonally as well as technically , the 598 was a waste of my time ......I am ready to be flamed by the fans now :P
Good luck MrGoat !