Most useful tweaks that are sensible and really make a noticeable improvement

Some of us are well beyond this now, but it never hurts to repeat the most cost-effective tweak you can buy is a tape measure to perfectly align your speakers exactly equidistant. My beautiful example of this is the time we were struggling to get good imaging in the Talon Audio room at CES. After more than an hour of tweaking I finally said let's forget the room and walls and just measure the damn things. Ten minutes later and even I was shocked how much better it sounded. Yeah! Science!

millercarbon"it never hurts to repeat the most cost-effective tweak you can buy is a tape measure to perfectly align your speakers exactly equidistant."

If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position. Do not tighten the clamp too tight or injury may result, which may explain some of the postings common recently on this site.

The only scientifically correct way to find the absolute best locations for speakers is using
the out-of-phase (speaker set up) track on XLO Test CD or similar Test CD or Test LP. All (rpt all) other methods, e.g., making speakers equidistant, making an equilateral triangle, using lasers, trial and error by listening, will be unable to find the absolute best locations. It would be like looking for a needle in a haystack. When the system is out of phase you should hear the sound coming at you from all around, with no particular direction. When that occurs, and it won’t happen overnight because the room acoustics if not treated interfere with how perfectly diffuse the sound can be on the out-of-phase track. Rome wasn’t built in a day. Anyway, be that as it may, when you get the out of phase track correct you will get the best sound when the system is IN PHASE. Everything else is a trade off. In some cases because the room acoustics are so bad, you might not even hear ANY differences when you move the speakers around. No one said it was going to be easy. Other locations might sound OK, but they won’t sound as good as the locations found using this out of phase method.

@clearthink

If you are going to do this for the purpose, goal, and objective of higher fidelity then you will also have to design, build, and install a clamping device to insure that your head never deviates from the optimum measured listening position.

I think what @millercarbon meant was making sure the speakers are equidistant from the back- and sidewalls. That will have a big impact on room acoustics regardless of the exact listener position.