Where do I start-amp or speakers ?

Ya, amp- speaker pairings are the most difficult pairings IMO,
and I know others agree.
. You have a huge room with those tall ceilings, more room for reverberations, but also more space to fill in with sound. I have learned to start with speakers that aren’t too extravagant so they can be matched with a good variety of amps at reasonable cost.
I have the Spatial M3 Triode Masters, price= $4000.

Now before anyone writes off these “Triode” Masters, these speakers don’t require vacuum tubes, and they don’t sound like tubes. They will fill the room with sound, they have two 12” cones with a voice coil on the upper cone. Very broad soundstage, SQ is equal to about an $8000 speaker, and that’s not hyperbole, b/c Spatial sells direct to the customer, so huge savings. Also, a 20 year warranty. Who does that these days? (Made in America too). Ok, I sound like an ad, I’m done.

Spatials are linear, and beautiful but punchy sound,and technically 16 ohm, but does require alot of power to drive them, more than just a small tube amp. One guy posted that he ran the Spatial M3 Triodes with the Schiit Freya and Schiit Vidar SS monos to great success. Vidar are like $7-800 each.
And if you already have the Freya, you’d be guaranteed great synergy using the Schiit products together!

Ive also ran the Spatials (and Dynaudio Contours, & Dali Helicon 400s) with the Primaluna Dialogue HP Integrated amp.
$4300.
Can run Triode or Ultralinear via remote. Can runs any tubes from EL34 to KT88 to KT150 for power section. 70 lbs. 3 huge transformers will give you the bandwidth you probably want coming from using the  Krell/Thiell combo.  This amp has twice as much power as needed for the Spatials, so you could also try the little brother, the Primaluna Dialogue Integrated, $3300

. I don’t know if you are open to tubes, but the Primaluna is another exceptional value, is not “tubey” sounding, is not soft, and is not slow.
This amp manhandles the Spatials, and can drive other speakers down to 2 ohms! 
So down the road, you could also try electrostats or other notoriously “hard-to-drive” speakers.

well, I already voted once: speakers first.

But then I reconsidered this thread and the OP original question.
Most ppl are responding, e.g, "I WOULD do this first .... " versus "I DID this first ...." . 

IOWs: theory vs reality. I'm not in the market for amps or speakers. I have 4 pair of speakers and 4 amps, 2 preamps that I am comparing to each other with different electronics: trying all permutations to compare. I enjoy this process, it is "fun" to me, tho may not be to others. 

My point is that if I were in the position of choosing speaker vs amp first, I wouldn't do either per se. I would do what I have already done: collect a few amps and speakers to compare rather than put all my eggs in one basket and buy "speakers first, then amp" or "amp first, then speakers". This method works for me. 

thinking back, I came to my present position by compiling my top 5-10 speakers I am interested in, irrespective of amp. Then make a list of 5-10 amps, irrespective of speakers. Then cross-match electronics, tube-vs SS vs class D, impedance, etc with the electronic/power requirements of the speaker. that will give a short list of amp-speaker pairings that would theoretically work together.

Then review the 'short list" for realistic considerations: the pairings may look good on paper, may seem good, but may not fly in reality. Like me, you might think to yourself, " I really think these speakers are the ones, but I'm not sure if amp A/B/C will produce the best sound ... also depending on the Preamp.

I'm just sharing how I have, over the last10 years, learned to mix and match gear, and avoid the pitfalls of "there is only one combo out there for me and my room". there are oodles of combos that will sound good in your room, but only A vs B vs C, etc comparisons will reveal the best sound your ear.