I know there are grainy garbage headphones like the MDR-7506, but when a decent one or an IEM is well EQ'd it can be a ridiculous improvement. Like say EarFun Free Pro 2 with Oluv's firmware EQ vs a Dusk doesn't show me anything more really. That said an LCD-5 impresses the hell out of me compared to the LCD-X's etc that I'm more used to. I don't think it's just about tuning, but when distortion etc are below audible levels then EQ can take you 99% there I'd say.

That said your ears are useless for EQ of that caliber so practically doesn't help much past a measured preset.