EQ APO is a free, open-source system-wide equalizer for Windows. Peace is a GUI frontend that makes it usable for non-engineers. Together, they let you apply AutoEQ profiles — community-measured corrections that bring any headphone closer to a target curve — in about ten minutes.
If you own a pair of Sennheiser HD 600s, applying the AutoEQ profile is genuinely revelatory. Same headphone, noticeably better sound. It's free. There's no good reason not to try it.