But that has nothing to do with quality or quality control, but rather with the circuitry/circuit technology used.
The circuitry and chip amplifiers used don't handle very low impedances very well, especially with power. Furthermore, these headphones often have a frequency-dependent impedance curve, which leads to an even lower impedance minimum and causes problems.
That's the price you pay for these very low-noise and high-performance circuits, and it only affects a very small portion of headphones on the market.
Transistor circuits like those in the H400 can have advantages in this regard, but they have other limitations.
I'm glad you're happy with them. I'd also like to hear the H400 and compare it.