The problem is that the drivers on cheaper headphones are less likely to be able to play frequencies other than the way they are tuned without having distortion. In other words, the driver is tuned to have the least distortion with the tuning the manufacturer has done on it. So if you change the frequency response using eq, you will be introducing lots of distortion to the driver. Whereas, with some headphones with better drivers, you can actually eq them and there would still be little to no distortion as the drivers on them are much more capable and dynamic. Basically, with the cheap ones, you will face a physical obstacle! The driver is simply not capable of moving the way you EQ it so the sound will be distorted.
I understand your frustration and I hope I have given you a more clear answer than the one you usually get from other people. Unfortunately, there are limits to EQ and you can nit achieve everything with EQ
Hans zimmer mountains