So how much do you think the placebo effect impacts our listening preferences?

IMHO the placebo effect happens more for subtle possible improvements than big ones. When something hits you obviously when you listen for the first time, that is when you know you have something. Unfortunately (or fortunately) many changes happen, many records get played, and it is very hard to know what or how much happened when.

Also, it is very hard to remember how something used to sound.

One other point - if you know something is better sounding than something else, by recommendation of someone you trust or universal acceptance, sometimes I'll just go for it knowing it is better. For example, I was told by Ron (Martin) Sutherland himself of the Phono Stage company and creator of Martin Logan several decades ago, that my phono stage would sound better with a linear power supply versus the switched one provided as standard. He had just started offering the upgrade of his trickle down technology from a phono stage costing 2.5 times as much as mine. It was a little less than half of what I got the phono stage for, but he said if it was his brother in law, he would tell him to do it, so I just did it. He even helped me install it so I wouldn't have to pay 2 way shipping to Kansas.

Could I tell afterwards? Not specifically, but I had also recently upgraded the interconnect from the PS to the amp, but sometimes knowing it is better sounding gives you peace of mind even if you didn't prove it specifically for yourself.

Perfect example - rollerblocks isolate components and also transfer vibration out of them. Inside them are ball bearings of certain specifications that make the box float. The new base level (formerly one top from the base) has 10 parts per million of imperfection in the shape/quality of it, and the highest level one, called a "super ball" (I kid you not), has 2.5 ppm, or a 75% reduction in imperfections. The cost of this upgrade is equal to the entire cost of the base rollerblocks with (the newly upgraded) standard bearings. Some people will buy the super balls based on the owner's recommendation knowing they are the best and have that peace of mind. Bear in mind that the previous improvement to the 10 ppm was down from 60 ppm, or an 83% improvement. Talk about the law of diminishing returns....This is the epitome of it, but he sells a ton of them for those who want the absolute best, and if I spent triple on my stereo,  I would probably get them. The owner did not recommend them to me based on my system, so he understands how high resolving a system you need to hear or appreciate the improvement. This is the case with fuses, power cords, etc.

I can't believe you guys are arguing if speakers are better than headphones. They are DIFFERENT and do different things.

I don't know anyone who prefers headphones unless they either have no decent stereo or can't play music out loud because it annoys someone.

A very strange twist from a discussion about the placebo effect.