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


This is not how it usually happens, but what if you simply must have a particular amp and then move with upgrades in both directions? Let's assume that the only source will be RTR deck, so no preamp will be needed.
The price range..how about $15k new or used ?
This is a hypothetical exercise, I am personally not prepared for that yet. However, it would be, theoretically, perfect minimalist audiophile set-up. 
I myself would probably choose either Lamm monoblocks or Gryphon stereo power amp, both used of course.
inna
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.