Here are some easy things
Walk around the room. Listen for certain frequencies changing volumes. If you hear nulls or peaks, sound treatment.
If your system has been as is for a while, take all the interconnects and speaker wire apart and clean with DeOxit or Stabillant 22.
Isolate speakers from floor. I used springs but my speakers have a big footprint.
Any old equipment in 20 to 30 yr range should be checked.
Those are low cost. Another quick one is the ADD POWR company. Most expensive one is $4000. It will just open up the music, make it clearer and has a 30 day audition.. Turn it on and play music. In 20 minutes, you will hear what it does. Turn it off to double check. Turn it back on, etc. Have fun.