the big one: how do you choose speakers? By what features, data?

You have to know exactly what you like, something you learn after a lot of experience which usually means you are entitled to a few missteps. I like line source dipoles which limits the search dramatically. Add to that a penchant for ESLs and I am down to just a few choices. 

If you are limited to smaller point source speakers the number of choices can be daunting. If you are looking for speakers that "disappear" you will be more likely to achieve that effect with mini monitors on stands adding subwoofers. The most difficult part of any speaker is the enclosure. It is very difficult to keep any box from becoming a musical instrument, but is much easier keeping a small enclosure from doing same. Just because they are small does not mean they are not capable of prodigious output especially when equipped with subwoofers and a proper 2 way crossover. Everyone should hear Roger's LS3 5As with subwoofers, a jaw dropping experience. 

Speakers are very simple devices that have to obey physical laws like any other transducer. The only other consideration that is as important is the room you are putting them in. This assumes your goal is accuracy, reproducing exactly what is in the recording and presenting it in a manner that could be thought of as realistic. There are certain types of speakers that because of their physical characteristics do this better than other types. It is these physical characteristics that one needs to pay attention to. 

@grislybutter These things do not necessarily show up in specs. They are usually design elements that produce a certain effect. Some of it is personal and a matter of experience. For instance, I could care less what a speaker looks like as long as it is well constructed. I want a speaker with sharply limited dispersion, no more than 45 degrees above 250 Hz. Only dipoles and horns meet this requirement. The purpose is to minimize room interaction. I prefer the larger, more realistic sound stage of a line source but in order to work well it has to cover the entire spectrum from 10 Hz to 20 kHz or beyond. This now limits me to very large dipoles. At one point Magneplanar was thinking of making an 8 foot 20.7. The marketing department talked them out of it! In order for a line source to function down to 10 Hz it has either got to be 60 feet tall or run from floor to ceiling, all the way from floor to ceiling. The two most common ceiling sizes are 8 and 9 feet which is why Sound Labs makes 8 and 9 foot speakers. You could easily make ribbons or planar magnetics do this but to my knowledge none do! Fine by me as I prefer ESLs anyway as they can be run without any crossovers to great advantage, excepting one to the subwoofers which I think are mandatory for these speakers to sound their best. As a matter of fact I purchased my Sound Labs without ever having heard or seen them. By design I knew exactly what they are capable of. I had 8 foot Acoustats for decades, a very similar speaker. 

@yxcbandit  The problem with trusting ones own ears is that most of us do not have ears we can trust. How you hear a system depends entirely on your experience listening to a multitude of systems. You cannot know what a system is capable of until you hear it. I built my first amplifier in 1967, a Dynakit Stereo 70. I had no idea how well a system could image until I heard the system of a high school teacher in Miami, FL in 1980. That system had been my target for some 35 years until I managed to build a system that functioned above that level. A large part of it was the room not his system. Back then people were just beginning to deal with room acoustics in residential settings. 

Another problem is your ears can not tell you what to do proactively. My ears do not tell me what equipment to buy. They do tell me whether or not I have that equipment set up correctly as they have that reference built in. 

The key is to gain as much experience as possible listening to other systems. This is not easy. I was fortunate in that I paid my way through school setting up HiFi systems for very wealthy people and worked with the top high end dealer in the area. I got to hear a lot of systems in more intimate environments. It is unfortunate that shows do not work well for this and you have to be deceptive with dealers. In order to get their best service you have to make them think you are qualified (read wealthy) and ready to spend a fortune. Wear gold jewelry and drive up in a 911.

In short, your ears can tell you what is acceptable to you. They can not distinguish  what a system is capable of without having heard it.  

@grislybutter , Marketing is the fine art of lying and numbers can be misleading. Learn about the physics of loudspeakers, crossovers and drivers. Learn about acoustics. Everyone is worried about ears. You need to use your eyes first. This is so and so a speaker, now how does so and so a speaker sound? People who know what they are doing can tell in a very general way how a speaker is going to react to the environment just by looking at them. This interaction between your speakers and the room they are in is the single most important determinant of sound quality.

Anyone can make a speaker that sounds good, but making a speaker that can image is a whole other problem and much more difficult. The Bose 901 is a great example of a speaker that many people love but it is the worst imaging speaker on the market, even worse than Tektons. The reason modern enclosed loudspeakers like Wilsons and Magicos are so expensive is that in order to make that kind of speaker work well you have to make perfectly silent enclosures and very complicated crossovers aside from using great drivers. 

@yxcbandit , Very good point about controlled dispersion and very true. The wall behind my SLs is covered with 4" acoustic tile. However, the best way to go about this is to measure first to see what your speakers and room are doing. Trusting your ears is a mistake if they have no reference. People who are use to bright loudspeakers will think speakers with a flat response sound dull. Learn what flat sounds like and then adjust the system to your taste. My system runs flat from 100 Hz to 20 kHz and the gets boosted at 6 dB/oct from 100 Hz down to 18 Hz. This gives the sensation of a live performance at more sane listening levels. Flat is the reference. With the newest digital preamp processors you have perfect control over all of this and you get a measurement Mic with the unit. For $300 you can get a wonderful USB mic and measurement program. Measure first then adjust to taste.

Because I know what flat sounds like I can tell what a system is doing in loose terms. I will never be as accurate as a calibrated microphone. Nobody will.

I have also seen some very silly acoustic treatments. Find your first reflections and place absorption there floor to ceiling. The first reflection pattern is not straight. It is the cross section of a sphere. Absorption needs to cover this entire pattern for standard dynamic speakers. People with dipoles only need to treat the wall behind the speaker. Why?  

@grislybutter , If you want to get serious I need to know how much you are willing to spend, the size and layout of your listening room and what kind of music you like. 

@grislybutter My 911 is Yellow! 

If imaging is your priority and with your situation there is only one way to go, The Harbeth P3ESR XD. Put them on stands exactly where yours are now and enjoy. Later you can add two subwoofers to the mix with a two way crossover. This lowers distortion in the little woofers of the P3 and raises headroom to an astonishing level. Close your eyes and you could easily imagine you are listening to a Magico or Wilson loudspeaker! Not Kidding!

I am wearing a Grateful Dead T shirt. 

@grislybutter Speed Yellow has a little red in it. It glows in the sun. On the road on a sunny day you can see it a light year away.

No! The only Harbeth you are interested in is the P3ESR XD. The small enclosure is almost like having no enclosure at all. Most enclosures including the larger Harbeths are musical instruments, they make their own sound. The P3ESR XD is the closest you can come to a Quad Model 57 and maintain your sanity. Do not let it's size fool you. The limiting factor for most speakers is the tweeter and the P3 has as tough a tweeter as any speaker. It only problem is handling bass which it doesn't. Two subwoofers are mandatory for the best performance, not by adding bass but by taking it away from those little woofers letting them be what they really are which is midrange drivers. That can be done down the line. Two of the little KEF Balanced Force subs make an excellent match. 

Genius? My wife would argue with that.