The most important determinant of sound quality is: wait for it. Pay attention now. Ears open. Ready? Here it is: Direct sound! Yes, direct sound trumps room reflections with respect to human perception of sound quality.
1.) Start with a speaker that gives flat FR. Use anechoic data to select for purchase a speaker with flat FR. What‘s that you say? You already bought a speaker, and anechoic FR isn’t flat? If the speaker you have does not deliver a flat FR in an anechoic chamber, then the best you can hope for is to try flatten, using EQ, based on measurements obtained in anechoic chamber. As close to flat as you can get. How? What guide? Use curve from anechoic measurement to EQ. Cut anechoic peaks. Boost anechoic dips. Mind the Q’s.
2.) Select for purchase speaker with smooth off-axis behavior, based on anechoic measurements. Now here, if your speaker does not have smooth off-axis behavior, you’re SOL. Because nothing, not even the most excellent room treatment, will pull your bacon out of the fire. Nothing can fix it. Best use poor off-axis speakers for a pair of headphones, because if you jiggle your head even a little bit, you’ve altered what hits your ears, to the point that FR is not smooth any more. These speakers can only give one tight little sweet spot where FR is not bumpy (same as saying smooth), and that is directly on-axis. Don’t sit too far away, and don’t move your head, because if you do, the room takes over. Well, there is other one fix: buy better speakers.
So, if my speaker‘s on-axis Klippel line is flat, and the DI is smooth, I’m golden? Yes! Physics will make such a speaker’s actual output in your room conform to a downward sloping curve. Which is, really, only what it looks like to your eye. What you’re truly after is not so much a treble cut as a bass boost. But boosting bass has its own challenges that are best dealt with separately. Because, if there’s anything I got out of calculus, it’s that a complex problem is best dealt with by breaking it into manageable parts.