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

The old real estate saw "Location,location location" applies to speakers but modify it to "Listen listen listen"

Ad copy and enthusiastic dealers can make you believe that brand X is the best thing since Rod Stewart sang standards at your dinner table. 

For me going out and doing the hard work of auditioning at the store and then at home with my gear in my room with my source makes some surprising conclusions. When you get to the "affordable" 10K speaker range they are all really good furniture, but they don't sound anything alike at your home.

Anecdotal story:  I had a pair of Vienna Acoustics towers that were at my home for a year. Really nice furniture indeed! And super cool see through cones (woohoo). I delivered them to a gearhead buddy and hooked up a pair of bookshelf KLH turds before my new Brystons arrived. What a hoot !  The Best Buy KLHs sounded "almost" as good as the Vienna towers at lower levels (muddy as dirt at a bit higher volume- no surprise there) Point being that price and sound are not directly corollary. 

Anyhoo...go on a listening tour with your most demanding source material then get your 2 favorites home and audition them there with you most demanding source material. No regrets afterward. Just wonderful music! 

Any speaker that don’t feel unstressed at 105db is out of the question.

Go listen at a dealer if you can. I was able to do that but a big part of my decision to buy was the universal praise from people on this forum and WBF. If so many think the brand you are looking at are great then some of them must have a room similar to yours and you can be confident the should work in your room. Good luck!!

You have received a lot of helpful advice above.  In particular, go listen at different shops and shows.  This is a learning experience, and the more you hear, the better you will become at discerning differences and the more confident you will be in your own judgment.  You will develop your own personal set of priorities.  I know my own priorities include speakers that sound lively and full at lower volume levels, not just when playing loudly.  That has steered me toward high efficiency models that tend to be more dynamic sounding such that they don't have to be cranked up.

Most of your auditioning should involve trying to replicate your home situation--speakers located similarly, you seated the same distance you would be at home, your own amp used for the audition, etc.  But, when you become experienced, you might also want to audition the speaker up close because "nearfield" listening reduces the influence of the room and you get to hear the particular characteristics of the speaker better and this is added knowledge--is the speaker well balanced?; are there annoying peaks and sibilance?; etc.

There are many high end speakers out there that deliver good imaging, reasonably good tonal balance, freedom from cabinet coloration, smooth sound, etc.  To me, the shortcoming of many, even very pricey, speakers is a lack of dynamics--it is not a case of not being able to play loudly, but a lack of realistic contrast between soft and loud such that one becomes disengaged from the music after a relatively short time.  Most of the truly dynamic speakers are high in efficiency because they don't require much current pushing through the voice coil to deliver a particular volume level.  The lower the current, the lower the heating of the wire and the less the signal is being compressed from increasing resistance in the wire as it heats up (thermal compression).  I have seen quite a few audiophiles have a "come-to-Jesus" moment when they first hear a very dynamic speaker system.  Many find the sound so compelling they jump on such speakers immediately.  I would suggest longer term listening because many such speaker have their own shortcomings--rough and peaking response, excessively hard-edge attack to notes, etc.  In other words, there are some possible tradeoffs that take some time to recognize.  

I agree that listening is truly the only way to accurately judge speakers. If not able to do so in your home then the dealers showroom might have to do. I have had/ have multiple speakers that I rotate in and out every few weeks, all that I personally think make music to my ears. You will know it when you find something that just gives you goosebumps when you listen, that makes your favorite music sound better than you have ever heard. Martin Logan CLS, Quad 63 and Wilson Watt Puppiesl are three that I love. The CLS’s I first heard at a shop when I was 18 and just couldn’t believe mysic could sound so good. Ditto for Wilson’s, excellent sound, accurate natural timbre and outstanding dynamics. Seek and you will find them eventually.