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
A pair of the Levinson ML-2 (25 watt) monos. Plus a pair of Quad 57's. Designed by Tom Colangelo. Each mono amp weighs 100 lbs. No skimping here on the power supply!
If you 'must' have a particular amp, that most importantly affects your choice of speakers, i.e. amp's power will need ____ efficiency speaker, in your space, at your desired max volume. 

Even if what you 'must' have is powerful, I always recommend efficient speakers so you can eventually try tube amp(s) within an affordable power range where there is more options.

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Pick your efficient speakers first! Within the 'efficient' category, find speakers you 'must' have. After speaker selection, knowing your space and volume preferences, you know how powerful an amp you will need. Then find an amp you 'must' have within that power category.

Enough power? IF not too much = less: money, heat, weight = more placement options.
The Levinson ML-2 mono amp has 1 Farad of capacitance in its cap bank! Something of a record here! And the ability to easily drive the 1 ohm Apogee Scintilla - a brutal load indeed! 200 watts @ 1 ohm!
Minimalist, but not totally:

I must have remote volume, mute, and remote balance from my listening position.to refine individual tracks the engineer’s didn’t get perfect. Or the equipment/system isn't getting it perfectly. Balance, enjoy now, check everything in the morning.

A very small balance adjustment can make a large difference to imaging, not just the center i.e. singer, but more distinctive localization of backup singers, other musicians, giving a more involving experience.

Another advantage: If you learn to listen for, appreciate distinct instruments, imaging, then when it is OFF, it is not easy to ’let it go’.

R2R, you have individual volume settings L and R, watching meters is one thing, and who knows when matching signal strength l/r are correct using the beginning of a program’s content. delivered sound to your listening position is the real thing.