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

@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.  

I'm still trying to figure out what all makes a speaker sound good to me. I can learn to like a lot of speakers, but I have found that when it comes to direct radiators I tend to like a 4" cone midrange, and 3/4" dome tweeter on a minimum baffle. Crossover up around 3500 Hz. Small bookshelves like this tend to have a great effect on me, often better than 2 ways with larger woofers, even though I can hear improved bass extension and lower distortion. Just add woofers/ subs to the little speaker and it's great! It's something about the dispersion characteristics of the smaller woofer and tweeter I suppose. 

For horns, I like full horn loading rather than hybrid designs, and smooth dispersion characteristics rather than full exponential loading like a Klipschorn of old. I think some degree of smooth off axis is taken into consideration on most modern horns now days.

In any case, I'm willing to try anything. If it works, it works. But if I were forced to buy without hearing I'd be looking for smooth on and off axis response with high efficiency and relatively easy impedance load. I'd be willing to sacrifice maybe some smoothness in response for better efficiency, so long as it's not too rough. I'm willing to sacrifice absolute impact / loudness for a smooth, natural and detailed sound. I don't like it really loud but I've experienced a sound I prefer more often with higher efficiency speakers.

@kokakolia looks cool but no OGY distributors in the US

"I have a 100Watt into 8Ohms amp but my room is small, so that won't matter much."

@grislybutter yes it will very much so. speaker sensitivity and amp power and current capabilities are critical or matching these two components and their working properly, regardless of room shape and size. the speakers and amp are working together no matter what room theyre in. 

Unless one is considering extraordinarily difficult to drive speakers and a requirement to play at extremely loud levels, I would expect 100 watts to be plenty, and, power is not a concern.  However, amps are critical to the sound of ANY speaker and quality of whatever watts it delivers matters a lot. 

I personally like low-powered tube amps the most, assuming they are used with compatible speakers, because they tend to be lively, engaging (keeps my attention), and at the same time do not sound edgy or hard and brittle.  Most high end, high-powered solid state amps are not offensive ((not "grainy" or "harsh" as some tube lovers claim, but, to me, they sound a bit lifeless and unengaging at lower volume levels.  I tend to not like most very high-powered tube amps because they have a hardness or "glare" I find a bit annoying in long-term listening sessions.