These days people (normal people, not people on this forum) use DSP to equalize. People here use cables and interconnects to equalize. I use a combination of placement, diffusers and measurements.
Having said that, I don't know if anyone actually likes ruler flat response? I don't think so. If you are remotely into headphones, they are all tuned in different profiles to cater to different preferences. I typically prefer warmth, as do most people.