Before you can fix anything, you need a measurement you trust. The good news: getting a usable first sweep doesn't require expensive gear — a calibrated USB measurement microphone, a laptop, and free software like Room EQ Wizard (REW) is enough to start.
Mic placement matters more than most people expect. For a general tonal-balance check, place the microphone at your primary listening position, at ear height, pointed straight up rather than at the speakers — this is the standard placement for whole-room averaged measurements.
Run a sine sweep at a moderate, consistent volume — loud enough to clear the room's background noise floor by at least 20dB, but not so loud you risk overdriving the speaker. Take 3 to 5 sweeps at slightly different mic positions within a foot of your main spot and average them; a single sweep can be misleading because room reflections vary a lot over small distances.
Once you have a clean average sweep, you have your baseline. Every EQ or room-correction change you make afterward should be compared back against this exact measurement — not by ear alone — so you can confirm a change actually helped rather than just sounding different.