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

Clarity and tight bass. Would also be nice,

If it had center synergy with my existing amp.

By how it looks.

@webking185 finally!

Two cents:   Try and figure out what type of speaker you like the sound of first. That really narrows down the list and ought to be the easiest thing to listen for. Our choices in design can change over time; that's fine as one upgrades.

Finding the 'house' sound that you prefer, of a good brand, within the type of loudspeaker, is worthwhile next (eg B&W, Focal, Dynaudio, Sonus Faber, et al). Finding a good used (within budget) pair of speakers as high up the line is great.

I am 100% into used. Most are taken care of very well, especially the higher up. As some say, these days, 'watts is cheap'. There are great amps with plenty of power, AB or D that don't break the bank. I have never allowed sensitivity to dictate my speaker choice and sound but I do usually dislike horn in-your-face sound so it never mattered. I did gradually move from electrostats to boxes.

Buying or upgrading speakers is always an interesting journey. Good work on doing your homework as best you can!

The CSS Audio 1TdX is really good. Reminds me a lot of Dynaudio C1s but will play louder without bottoming out. They have a bass module that is very reasonable that turns it into a pretty badass 3 way for pretty cheap ish. If I would have known that before I sold them I may not have.