I should start with speakers. This I know. I’d like to establish a new system.

@tomcarr  Agreed. 

If I was in the position of the OP, I do the following:

1. Try to size up the assets and challenges of the acoustics of the room. 
I'd lay hands on whatever gear I can find to listen with.
I'd listen, position, and once some reasonable positioning is established, I'd measure for frequency response. 

2. With the information gained above, I'd try to treat the rooms major flaws, focusing first on 20hz to 300 hz, since those frequencies have to be mastered before dealing with the rest of the range. I'd work a bit, next, on 300hz-20khz.

3. Having brought the room to a baseline of decent response, I'd do a bit of research on speakers and amps that will match my tastes and my room. 

4. Next, I'd start borrowing speakers or buying, used. I'd get a decent solid state amp (used, hopefully a keeper) that can drive a bunch of speakers. 

That would be the "launch sequence" to getting the basis of a good system going, in my opinion. 

Sum:

• Get the room ready.
• Try speakers and amp.
• Get final speakers.
• Get final amp.
• Figure out how to optimize the rest (preamp, DAC, cables, power, additional treatments, subs).

@hilde45 

Bravo!

Fantastic summation.

Orderly progression.

Now THAT'S how to set up a dedicated room!

Of course it depends on what you value most; smooth rich sound w/ nice subtle details often w/ good 3 D imaging or more live, dynamic sound that can rock out without straining or requiring heroic amplification. Very challenging to find both in speakers that cost under $20K. You’ve got a big room so if you like to play loud & enjoy music w/ real bass, a high sensitivity speaker may be your only choice in your price range. Volti Rivals work very well for me in a big room w/ good tube amplification (Rogers High Fidelity). 

@hilde45 A local guy to me is selling an almost new Blade 1 Meta for $18k. His ad and 2 others for under $20k are on USAM.

I was tempted to buy since the Blade has been my fav speakers for about a decade. However, I decided that the Yamaha NS5000 I own now is something I want to stay with.

Only issue with the Blade for the OP is that the Op’s power amp maybe too weak for the Blades. I would pair the Blades with either the $2k Schitt Wotan or the $20k CODA #16. Both of which I own. Those 2 amps have prodigious power. The Wotan is not at the sonic levels of the #16 (but close). It is at the level of many apps that are considered top notch. Schitt also have a 15-day home trial.

@knotscott 

+1 Yep, look at contemporary speakers. 

I would review Stereophile, The Absolute Sound, and HiFi+ recommended component issues. Concentrate on descriptions that strike you as your liking in sound quality. 

Then get on the road. Make appointments with a couple dealers in advance to audition the best fit two or three. Find the one with the emotional connection. Not just that has the most detail and best slam.