Building new system starting with power amp. What would you choose ?

Within that budget, I would be looking at a pair of Coincident 211PP Dragon MK II monoblocks:
http://www.coincidentspeaker.com/211PP-Dragon-MkII-Mono-Amp.html

My latest video on the DIY HiFi Life Channel covers the VTV Purifi Amplifier. Stay tuned for my next video where I show how to upgrade this amp as well. Like and Subscribe is always appreciated.
https://youtu.be/mBj8EjHXBJE

CODA Continuum or Ts. 
Pass Labs

I have Pass xa30.8 Wilson Sabrina and a Nagra 300i with some B&w both really nice

What is also really nice is the Devialet amps a mix between A and D class

Wavac EC300b

Pass Labs INT250.  It is integrated at 250 W/channel RMS and nothing else will come close.

"Within that budget, I would be looking at a pair of Coincident 211PP Dragon MK II monoblocks"

I would do what @nordicnorm says.  In fact, I did.

Andrew

Depends on the speakers. I could suggest if I knew which speakers  you are using 

Coda full class A amps are hard to beat.  

Again, depends on your speaker choice.

Respectfully,
Jose

Once again, the tail wants to wag the dog.

Engineering is all about achieving an optimal result within the constraints you have to work with.   Constraints are more than just budget, though that is the most obvious.  How big is your room?  What kind of music do you listen to?  How loud?  Decoration is an issue and of course, there's WAF to consider for many.

So starting with the power amp and going from there is a useless question.

That being said, with $15k to play with, I'd get a pair of VTA M125 monoblock tube amps at 120 watts each and look for a used pair of Avalon Eidolon loudspeakers.  With care you could do all of that for $15k.

Odyssey Audio Stratos Exstream mono blocks, Symphonic Line Kraft 300 dual mono Amp.

If you are a Tube kind of guy, then try the TriangleArt M100's just for the heck of it. I believe you will be amazed at what you will get from them.
Call and talk to Tom, he will set you up.

Because just starting out you have no idea what you really like. It will prob take years to grow, learn as evolve as you better understand what you like and what products offer what. 
Amp- I would start my buying used because if you don’t like you can resell for close to what you paid as you learn. Stick with top notch brands that hold their value. 

I agree with 'unsound'.....you've got it backwards. Choose the speakers that fit your taste and then find amplification that is compatible with the speakers

I just built a new system a little over a year ago. I went with a mcintosh mc462 and am very happy with it.

D-Sonic, D-Sonic, D-Sonic.
https://www.d-sonic.com/amplifiers/