First, get REW (it's freeware) and a calibrated microphone (e.g. MiniDSP umik-1). Take in-room measurements for L and R channels independently at your listening position. These and other measurements will reveal much about your acoustics that you may not be aware of presently. It's a no-brainer.
With those measurements, you can compare your response to the Harman and NAD target curves (they're very similar), and you will have a solid idea of what to do with EQ settings. Without those baseline measurements that serve as your point of reference, you're playing "Pin the Tail on the Donkey." If you know what that is, then you'll understand the analogy. REW allows one to dial in the appropriate settings for each channel.
I used a dbx 2231 graphic EQ for a long time with very good results, which is an economical and reasonably transparent platform with 31 octave resolution. Accuphase used to make a nice graphic EQ (G18), and examples in good working order remain expensive. There are boutique, studio quality graphic and parametric EQs that are very good, some being certainly better than others. The downside is they all insert another cluster of pedestrian electronics and additional connections/cables into the signal path, and some degradation (e.g. soundstage) tends to become apparent if the system is resolving enough.
If you use a digital platform that supports inserting a text parametric EQ files for headphone or speaker correction, then you are in luck because REW can use your L and R measurements with a target curve of your choosing to generate PEQ files that will put you on target. I eventually found this to be the superior in terms of integrity and sound quality and I cannot see myself ever returning to a hardware EQ.
